fix(opencode): lock-specific error detail and serve-process observability on startup failure #20

When opencode serve fails at startup, the turn.start.failed detail is a raw child-stderr blob ('OpenCode server exited before startup completed (code: 1)...'). Nothing records attempt count, concurrent serve process count, or a lock-specific hint, so each recurrence requires live-DB forensics to classify.

Proposed: include attempt count and a 'database busy, retry' hint in the failure detail for SQLITE_BUSY-class exits, and log concurrent opencode serve process count at failure time.

Diagnosed from live-DB forensics; agent harness openrouter/meta/muse-spark-1.3-contributor via T3 Code.