Adds opt-in `auto_archive` flag to `task_plan_publish`. When set, the
parent spec change three-way-merges and archives automatically after the
last sub-task completes via `task_plan_complete_subtask`. Default off
because silent state change after the final completion is risky if the
merged spec has not been verified — opt in per plan once the lane lands
cleanly. Conflicts on the three-way merge are non-fatal: the completion
still returns `status: 'completed'`, the archive is skipped, and a
`plan-archive-blocked` observation is recorded on the parent spec task.
Other auto-archive failures (missing CHANGE.md, write errors) are
recorded as `plan-archive-error` observations. The completion response
now carries an `auto_archive: { status, reason?, archived_path?,
merged_root_hash?, applied?, conflicts? }` field on every call. New
observation kinds: `plan-config`, `plan-archived`, `plan-archive-blocked`,
`plan-archive-error`.

Also fixes a latent race in `@colony/core` `readSubtask`: when claim and
complete observations share the same millisecond timestamp, SQLite's
`ORDER BY ts DESC` had undefined tie-breaker behavior. Status is now
resolved with terminal-state-wins precedence so a completion is
authoritative once it exists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>