fix(runners): discoverCcAgents fallback timeout has no default for execFile

[OneStepAt4time]

Problem

In src/runners/cc-agents-discovery.ts, the discoverCcAgents() function uses options.timeoutMs ?? DEFAULT_TIMEOUT_MS (line 141) for the timeout option to execFileAsync, but DEFAULT_TIMEOUT_MS is defined as 10_000 (10s).

However, detectCcVersion() (line 102) has its own hardcoded timeout: 5000 (5s). If discoverCcAgents is called with timeoutMs: 3000, the version check still takes up to 5s, which exceeds the caller's intended timeout.

Additionally, there is no signal-based cancellation — if the claude agents --json command hangs beyond the timeout, the process is killed by Node but error handling assumes a clean message.

Fix

• Pass timeoutMs through to detectCcVersion as well, or derive version-check timeout from the caller timeout
• Consider using AbortController for cancellation propagation

Impact

• Low — unlikely to cause production issues at current timeout values
• But the inconsistency will be confusing when feat: multi-agent support — strategic gap exposed by OpenACP (28+ agents) and cc-connect (10+ agents) #3180 adds dynamic runners with different timeout expectations

Discovered during

Code quality sweep on src/runners/ (pre-#3180 audit).

[OneStepAt4time]

Resolved by PR #4547 (merged on 2026-06-01). timeoutMs now properly passed to detectCcVersion in discoverCcAgents.