Plugins could observe session/tool/approval lifecycle but had no way to
observe kanban task transitions. Adds three observer hooks fired by the
board's claim/complete/block transitions:
- kanban_task_claimed (dispatcher process, before worker spawn)
- kanban_task_completed (worker process, carries summary)
- kanban_task_blocked (worker process, carries reason)
Each fires AFTER the DB write txn commits, so a plugin observes durable
state and a slow/hanging callback can never hold the SQLite write lock.
All firing is best-effort: a raising hook is logged and swallowed and
never breaks a board transition. profile_name is resolved from
HERMES_HOME so dispatcher- and worker-side hooks carry the right profile.
Requested by @Smithangshu on Discord.
