feat(gateway): add context provider abstraction with LINE buffered context

docs/gateway-context.md

@@ -0,0 +1,44 @@
+# Gateway Context Providers
+
+OpenAB gateway adapters can optionally enrich an admitted user turn with recent chat context that the bot would otherwise miss because of mention gating or platform-specific admission rules.
+
+The gateway-level `ContextProvider` abstraction keeps this behavior shared across platforms:
+
+- `observe()` records a message that was seen by an adapter but not dispatched to the agent.
+- `fetch_context()` returns recent context for an admitted turn.
+- `inject_context()` prepends that context with a clear boundary before the current message.
+
+## Provider Types
+
+| Provider | Intended platforms | Data source |
+|---|---|---|
+| `BufferedContextProvider` | LINE, Telegram, WeChat/WeCom, Feishu fallback | webhook observe -> local bounded buffer |
+| `ApiFetchContextProvider` | Discord, Slack, Teams, Google Chat where available | platform history API |
+| Hybrid provider | Google Chat and other mixed-permission platforms | API fetch when possible, buffer fallback otherwise |
+
+The first implementation wires LINE group/room text to `BufferedContextProvider`. Future adapters can reuse the same trait without changing the prompt injection format.
+
+## Defaults
+
+Context capture is disabled by default.
+
+| Setting | Default |
+|---|---|
+| `enabled` | `false` |
+| `ttl` | `24h` |
+| `max_messages` | `50` |
+| `max_chars` | `8000` |
+
+Gateway-wide environment variables use the `GATEWAY_CONTEXT_*` prefix. Platform-specific settings can override them; for example LINE uses `LINE_GROUP_CONTEXT_*`.
+
+## Scope And Storage
+
+Buffered context is:
+
+- scoped by platform, channel, optional thread, and bot id
+- in-memory only
+- drained after injection
+- bounded by TTL, message count, and total characters
+- not long-term memory, retrieval storage, or GBrain state
+
+This is intentionally short-term conversational continuity. Platforms with reliable history APIs can later implement API-backed or hybrid providers instead of relying only on local buffers.

docs/line.md

@@ -83,7 +83,7 @@ In the LINE Developers Console → **Messaging API** tab → scan the QR code wi
 
 - **1:1 chat** — send a message to the bot, get an AI agent response
 - **Inbound voice messages in 1:1 chat** — LINE-hosted audio messages are downloaded through the LINE Content API and forwarded to OpenAB as `audio` attachments, so the existing STT flow can transcribe them. This requires `[stt] enabled = true` in OpenAB core. See [STT (Speech-to-Text)](stt.md).
-- **Group chat** — add the bot to a group; it responds only when @-mentioned (see @mention gating below)
+- **Group chat** — add the bot to a group; it responds when directly @-mentioned. Deployments can opt in to folding recent unmentioned text into the next direct-mention turn through the gateway ContextProvider buffer (see @mention gating below)
 - **Inbound images** — user-sent LINE images are downloaded through the LINE Content API and forwarded to OpenAB as image attachments
 - **Webhook signature validation** — HMAC-SHA256 via `LINE_CHANNEL_SECRET`
 
@@ -96,11 +96,12 @@ In the LINE Developers Console → **Messaging API** tab → scan the QR code wi
 
 - **Threads** — LINE has no thread/topic concept. All messages in a chat share one agent session.
 - **Reactions** — LINE Bot API does not support message reactions.
-- **@mention gating** — Supported (zero-config). In group/room chats the gateway only forwards messages where the bot is explicitly @-mentioned (LINE's native `mentionees[].isSelf` signal). 1:1 DMs are always forwarded. No env var is needed.
-  - *Limitation — non-text messages*: LINE only attaches mention data to text messages. Images, videos, stickers, files, and location messages in groups are silently dropped because they cannot carry an @-mention.
-  - *Limitation — group voice messages*: LINE voice/audio messages in groups and rooms are also dropped today because audio messages do not carry mention metadata. This PR only enables inbound voice STT for 1:1 chats.
+- **@mention gating** — Supported (zero-config). In group/room chats the gateway only dispatches a visible bot reply when the bot is explicitly @-mentioned (LINE's native `mentionees[].isSelf` signal). 1:1 DMs are always forwarded. No env var is needed.
+  - *Optional short-term text buffering*: when `LINE_GROUP_CONTEXT_ENABLED=true`, unmentioned **text** messages in groups/rooms are observed by the gateway ContextProvider and buffered locally for up to 24 hours. The next directly @-mentioned text turn for the same chat drains that buffer and prepends it as short-term context. The buffer is capped per chat by message count and total characters. This improves conversational continuity without making the bot reply to every group message.
+  - *Limitation — non-text messages*: LINE only attaches mention data to text messages. Images, videos, stickers, files, and location messages in groups are still dropped when not directly @-mentioned because they do not enter the short-term text buffer.
+  - *Limitation — group voice messages*: LINE voice/audio messages in groups and rooms are also dropped today because audio messages do not carry mention metadata. LINE inbound voice STT is currently for 1:1 chats.
   - *Limitation — `@All`*: A group-wide `@All` mention does **not** trigger the bot; only a direct `@BotName` mention does.
-  - *Breaking change*: This gating is always active. Deployments that previously relied on the bot responding to all group messages will need to @-mention the bot after upgrading.
+  - *Behavior note*: the short-term context buffer is local, bounded, temporary, scoped by platform/chat/thread/bot, and drained after injection. It is not a long-term chat archive or GBrain memory store.
 - **Markdown rendering** — LINE uses its own text formatting. Agent replies are sent as plain text.
 - **External-content images** — LINE image messages backed by `contentProvider.type = "external"` are not downloaded yet.
 - **External-content audio** — LINE audio messages backed by `contentProvider.type = "external"` are not downloaded yet.
@@ -111,6 +112,13 @@ In the LINE Developers Console → **Messaging API** tab → scan the QR code wi
 |---|---|---|
 | `LINE_CHANNEL_SECRET` | Yes | Channel secret for webhook signature validation |
 | `LINE_CHANNEL_ACCESS_TOKEN` | Yes | Channel access token for Reply/Push Message API and LINE-hosted image/audio downloads |
+| `LINE_GROUP_CONTEXT_ENABLED` | No | Opt in to buffering unmentioned group/room text for the next direct mention. Falls back to `GATEWAY_CONTEXT_ENABLED`. Default: `false` |
+| `LINE_GROUP_CONTEXT_TTL_HOURS` | No | Hours to keep unmentioned group/room text eligible for the next direct mention. Falls back to `GATEWAY_CONTEXT_TTL_HOURS`. Default: `24` |
+| `LINE_GROUP_CONTEXT_MAX_MESSAGES` | No | Maximum buffered unmentioned text messages per group/room. Falls back to `GATEWAY_CONTEXT_MAX_MESSAGES`. Default: `50` |
+| `LINE_GROUP_CONTEXT_MAX_CHARS` | No | Maximum total buffered text characters per group/room. Falls back to `GATEWAY_CONTEXT_MAX_CHARS`. Default: `8000` |
+| `LINE_CONTEXT_BOT_ID` | No | Stable bot identity used for ContextProvider isolation when multiple bots share a LINE group. Falls back to `LINE_BOT_ID`, then `line-default-bot` |
+
+See [Gateway Context Providers](./gateway-context.md) for the shared context buffering model and future API-fetch/hybrid provider direction.
 
 ## Troubleshooting
 

gateway/Cargo.toml

@@ -14,6 +14,7 @@ reqwest = { version = "0.12", default-features = false, features = ["rustls-tls"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 anyhow = "1"
+async-trait = "0.1"
 uuid = { version = "1", features = ["v4"] }
 chrono = { version = "0.4", features = ["serde"] }
 hmac = "0.12"