A session created in the wrong directory needs its cwd corrected after the
fact. session.cwd.set only reaches live runtime sessions, so cold rows were
stuck. The new RPC targets the persisted row by session_key, validates the
folder, and REPLACES the git branch/root identity (update_session_cwd grows a
replace_git_meta flag) so the project tree's grouping follows the move instead
of pinning the session under the project it left via a stale git_repo_root.
A live idle agent bound to the row is re-anchored through the runtime path;
a mid-turn session refuses with 'session busy'. Runs on the RPC pool — the
git probes are subprocesses.
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