bug: csa review silent failure — zero output on subprocess crash

[RyderFreeman4Logos]

Summary

When a csa review subprocess crashes (e.g., SIGTERM, EPIPE), the caller receives zero stdout output and only a generic error message. The subprocess's stderr (containing the actual crash details) is lost because the pipe is already closed when the crash occurs.

Related

Companion to #272 (recursive invocation root cause). This issue focuses on the observability problem: even after #272 is fixed, future subprocess crashes will be equally opaque without better error reporting.

Current Behavior

$ csa review --branch main
# stdout: (empty)
# stderr: "Error: Execution interrupted by SIGTERM"
# exit code: 143 or 144

The actual crash information (Node.js EPIPE stack trace, claude-code-acp error details) is written to the subprocess's stderr but never reaches the caller because:

1. CSA sends SIGTERM to the subprocess
2. Subprocess tries to write error to stderr → EPIPE (pipe already closed)
3. CSA only reports the SIGTERM, not what caused the subprocess to hang/fail

Expected Behavior

$ csa review --branch main
# stderr should include:
# Error: Subprocess 'claude-code' crashed with EPIPE
# Last 10 lines of subprocess stderr:
#   node:events:486  throw er;
#   Error: write EPIPE at ...utils.js:50:28
# Session: 01KJEVXNPXJSK8JD18AQVPHP27

Suggested Fix

1. Keep subprocess stderr pipe open until after the process fully exits
2. Capture the last N lines of subprocess stderr before reporting the error
3. Include subprocess stderr in the session metadata for post-mortem analysis
4. If subprocess exits non-zero, always include its stderr in the error report

Environment

• csa 0.1.43 (v0.1.15-38-g718232d-dirty)
• Linux 6.1.0-43-amd64

[RyderFreeman4Logos]

Fix Applied in PR #275

Root Cause

AcpError::ProcessExited was a tuple variant carrying only the exit code (i32). When ensure_process_running() detected a dead subprocess, the captured stderr buffer (containing crash details like Node.js EPIPE stack traces) was silently discarded.

Fix

Changed ProcessExited from ProcessExited(i32) to ProcessExited { code: i32, stderr: String }. The ensure_process_running() method now calls self.stderr() to capture the last 10 lines of subprocess stderr and includes them in the error message.

Added a format_stderr() helper in error.rs that formats the stderr output only when non-empty.

Files Changed

• crates/csa-acp/src/error.rs — new struct variant + format_stderr() helper
• crates/csa-acp/src/connection.rs — ensure_process_running() now captures stderr

Example Output (After Fix)

ACP process exited unexpectedly: code 143; stderr:
node:events:486 throw er;
Error: write EPIPE at ...utils.js:50:28