protocol: run a protocol as a committed plan, with a log of how it ended ...

A protocol was only text until now. The new `protocol` tool starts one: its
steps are spliced into the session's todo plan after the agent's current
position (nothing already done is reordered), a row appears in `protocol_runs`
with a snapshot of the version, steps and post-checks, and the run stays
`running` until it is closed.

`finish` refuses while any step of the run is open, refuses outright when one
failed, and requires a report — it is the run's record for the user. `cancel`
and `fail` require a reason and mark the rest of the run's steps `skipped`, so
the plan does not keep work that was never done. One run per session is active
(a partial unique index enforces it).

The commitment also has to survive the context: every model turn while a run is
open carries a system reminder with the protocol, its steps and the way to close
it, and the final-turn intercept now fires for an open run even when the todo
shows no open steps, so a bare-text turn cannot leave one running.

`protocol` is wired into assistant, developer, discuss and server_admin next to
`skills`; sub-agents get neither, because a protocol belongs to the session's
plan. The run log has no UI yet — `protocol(op='status')` is the current view.

todo: give plans their own table and recover them after a crash ...

Todo already survived restarts inside the generic session_store KV row, but
the plan there was opaque JSON with no timestamp: after a server crash the
agent resumed a step marked in_progress with no way to tell whether it had
ever been done.

session_todos (user_id, session_id, tasks JSONB, revision, updated_at) now
holds the plan. Existing rows are migrated on first use and the KV source is
deleted in the same step, so clearing a plan is not undone by the next
process start.

Each step gains provenance fields (source, step_id, protocol_run_id) for the
protocol work, and insert_steps() splices a block after the step the agent
has already reached instead of making it re-issue the whole plan.

get_progress_message() now flags a plan whose in_progress step predates this
process once per (session, revision): re-check the real result before
marking it done.

The todo op "clear" now uses the same user scope as load/save; it used to
prefer ctx.user_id and could delete a different row than the one written.