pi-subagent: opt-in sessions, to follow up, resume after a timeout, or fork

run-agent.sh still runs with --no-session by default. --session saves the
run as session <name> under <workdir>/sessions/; --followup TEXT sends the
next message in it, from any provider and model; --fork FROM starts <name>
as a copy of FROM. A follow-up or fork inherits --tools, --thinking, --cwd
and --context-files, so a read-only subagent stays read-only, and the
earlier round's files move to <name>.round<N>.*.

Sessions are opened by file path: by id, pi looks only among sessions
started in the current directory and otherwise silently creates an empty
one, exit 0. A fresh retry moves the old session aside, since pi would
resume it given the same id.

pi-review-panel launches members with --session, so a contradiction can be
put to both reviewers before it is settled; verdicts are still tallied from
the original reviews.

From dot request 20261021-123436-work-laptop-d0adf2.

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