fix(daemon): preserve ACP startup exit diagnostics

[Siri-Ray]

Problem

When an ACP child exits during initialize or session setup without returning a JSON-RPC error, the bridge emits only a generic exit message. Stderr is retained only for an AMR-specific classifier, so startup failures from adapters such as Hermes and Kimi lose the actionable provider/configuration diagnostic.

Expected behavior

Attach a bounded, redacted stderr tail and the active ACP phase to a structured child-exit error. Keep the user-facing message concise and never expose raw secrets or terminal control sequences.

Verification

Add regression coverage for startup-phase classification, ANSI removal, and secret redaction.

[lefarcen]

Hey @Siri-Ray — thanks for writing this up so clearly.

The missing stderr tail during initialize / early session setup is exactly the part that makes these ACP startup failures hard to debug, and the constraints you called out here make sense: bounded diagnostics, active-phase context, ANSI stripping, and secret redaction.

I also see PR #6267 open for this already, so we can track the implementation there while keeping this issue as the problem statement.