feat(ui): show sessions_spawn label instead of agentId

[JimmyBlanquet]

Summary

Make the UI prefer the human-readable label field passed via sessions_spawn over the technical agentId / sessionKey UUID. Falls back gracefully if label is missing.

The OpenClaw gateway already accepts and stores a label field in sessions_spawn (e.g. "Analyste de portefeuille") and exposes it via sessions.list. The UI was reading it in chat-dock-store.normalizeSession and on the chat page list, but a few prominent UI surfaces still showed only the technical session key. This made multi-agent demos hard to follow because every spawned session appeared as agent:main:subagent:5533959a-... instead of its human name.

Files changed

• src/components/chat/SessionSwitcher.tsx — the dropdown items and the active session display name now use session.label first via a new formatSessionDisplay helper. Previously it only called formatSessionName(session.key).
• src/components/pages/ChatPage.tsx — formatSessionTitle (used in the chat page header) now accepts a sessionLabel arg and prefers it over the agent-name lookup. The single call site passes the active session's label from the sorted list.
• src/hooks/useSubAgentPoller.ts — harden toSubAgentInfoList against empty/whitespace label values (s.label?.trim() || ...), keeping the existing fallback chain (Sub-<uuid> → agentId) for backwards compatibility with the test suite.
• src/store/office-store.ts — in processAgentEvent's two pending-sub-agent branches, use the actual agentId as the temporary label instead of Sub-<8-char slice>. The sessions.list poller still overwrites this with the real gateway label as soon as it arrives, but the visible-during-race string is now meaningful.

Behavior

• When sessions_spawn is called with label: "Analyste de portefeuille", both the SessionSwitcher dropdown and the chat page header now display Analyste de portefeuille… (truncated at 24/32 chars) instead of subagent:5533959a-1a5e-….
• When no label is provided, the existing behavior is preserved (agent name for agent:foo:main, formatted suffix otherwise).

Test plan

• npm run typecheck — passes (no new TS errors)
• npm test — 475/475 tests pass, including the existing useSubAgentPoller label fallback tests
• npm run build — passes (vite build produces all expected chunks)
• Manual: run with a real gateway, call sessions_spawn with a label, observe the SessionSwitcher and chat page header show the label
• Manual: spawn without a label, observe fallback to formatted session key (no regression)

Notes

• I did not touch src/i18n/locales/ (concurrent FR translation work in another branch).
• The fallback chain in useSubAgentPoller was deliberately kept as label → Sub-<uuid> → agentId rather than label → agentId → Sub-<uuid> because the parent agent's agentId (e.g. "main") shouldn't be displayed as the sub-agent's identity, which an existing test (subagent-poller.test.ts:124) explicitly enforces.

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