fix(staged): keep ACP agent alive by pinning its dir on PATH + log stderr

[matt2e]

Problem

Starting a Claude ACP session in the staged app could fail with an opaque error:

ACP init failed: Error { code: -32603: Internal error, message: "Internal error", data: Some(String("server shut down unexpectedly")) }

The agent subprocess was dying before responding to the initialize handshake. Root cause: claude-agent-acp is a #!/usr/bin/env node script, and the agent is exec'd inside the session's working directory through a clean-env (env_clear) login shell. If that directory's shell init doesn't put a compatible node on PATH (Hermit/direnv can replace PATH, nvm may not activate), the shebang fails with env: node: No such file or directory (exit 127) and the process exits instantly. Its stdout closes before the initialize response → the client reports "server shut down unexpectedly".

It was hard to diagnose because the agent's stderr was sent to /dev/null, hiding the real env: node: ... message.

Not a version issue: claude-agent-acp 0.39.0 is the latest, and the negotiated protocol version (ProtocolVersion::LATEST = V1) matches what the agent speaks.

Fix

In crates/acp-client/src/driver.rs:

1. Pin the agent binary's directory on PATH for the exec. The exec line is now exec env PATH=<binary_dir>:"$PATH" <binary> <args>. Since the resolved binary lives in a node bin dir that also contains node, this guarantees the shebang's interpreter is always found regardless of what the session dir's shell init does. Falls back to the plain form if the binary has no parent dir.

2. Stop discarding the agent's stderr. Changed Stdio::null() → Stdio::piped() and drain it line-by-line into log::warn! (tagged with agent label + session id). Future spawn/shebang/shell-init failures are now visible instead of vanishing.

Verification

• cargo check and cargo test -p acp-client pass (16 tests).
• Reproduced the original failure (env: node: No such file or directory, exit 127) with node stripped from PATH, then confirmed the new exec line negotiates initialize successfully under the same stripped PATH.

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