Add debug logging for JSON parse errors in ACP handler

[Copilot]

The ACP handler silently ignores JSON parse errors when processing messages line-by-line (line 121). This makes protocol issues difficult to diagnose during development.

Changes

• ACP handler: Add environment-gated debug logging (DEBUG or WORK_DEBUG) that emits parse errors and problematic lines (truncated to 200 chars)
• Tests: Add 4 unit tests covering debug/non-debug modes, both env vars, and line truncation with proper isolation

Usage

DEBUG=1 work notify send TASK-123 to acp-target

Output when invalid JSON is received:

ACP message parse error: Unexpected token 'i', "invalid json" is not valid JSON
Problematic line: invalid json

Normal operation unaffected when env vars not set.

Original prompt

Enhance the ACP message handling by adding debug logging for JSON parse errors. Currently, the acp-handler.ts implementation ignores parse errors for partial messages (line 121). The PoC implementation reports these errors for debugging (refer to dev/poc-opencode-server/complete-acp-test.js:35-39).

Proposed Change
Introduce debug logging for JSON parse errors, enabled by environment variables DEBUG or WORK_DEBUG, without changing normal operation. Debug logs should identify the error details and the problematic line. Here's the suggested change:

try {
    const msg = JSON.parse(line) as ACPMessage;
    this.handleMessage(msg);
} catch (err) {
    // NEW: Log parse errors in debug mode
    if (process.env.DEBUG || process.env.WORK_DEBUG) {
        console.warn('ACP message parse error:', err instanceof Error ? err.message : String(err));
        console.warn('Problematic line:', line.substring(0, 200));
    }
}

Benefits

1. Easier diagnosis of protocol issues during development or debugging.
2. Normal operation remains unaffected since logging is environment-controlled.
3. Aligns with the PoC debugging pattern.

References:
PoC: dev/poc-opencode-server/complete-acp-test.js:35-39 (or updated PoC location that reflects related/debugging concepts).
Proposed update target: src/core/target-handlers/acp-handler.ts:118-123.

This pull request was created from Copilot chat.

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