Harden bored execution: spawn lifecycle + structured event logging (#10.1)

[0xbeckett]

Locks down the spawn lifecycle and adds full structured observability to bored's execution path.

All 7 acceptance criteria pass with tests:

• orphan process reaping on spawn failure/exit
• timeout events emitted with structured payloads
• abort receipts on cancellation
• jsonl event feed
• query API over the event log
• stable machine-readable error codes
• worktree cleanup wired through the process adapter (this was the gap review caught cycle 1 — worktrees were leaking in the prod path; now reaped + covered by a test)

Survived multiple daemon restarts mid-run without losing progress. Requested by Jason (frgmt0). Internal ticket OPS-168.

Note: originally pushed to the 0xbeckett/bored-hardening mirror (0xbeckett/bored-hardening#1); re-landed here on frgmt0/bored directly per request — that PR can be closed.