hub: a supervised session counts as not listening once its local-mode runner stopped (§5.3 after #1294)

tsk-mcd2xwwa2fzv open

Follow-up to #1294 (runner local-mode contract, tsk-qe0ka9e341gh). The contract (§5.3, §15.11, "At the hub") exempts a supervised session from member_not_listening only while an active placement names its credential (placement.credential_id == credential.id). Today hub/src/maity_hub/not_listening.py (_excluded) still exempts every supervisor session, and hub/tests/test_not_listening.py asserts that.

To do:

Exempt a supervised session only while such an active placement exists; otherwise it can become "not listening" like any other session.
Flip the existing test, add one for each case (active placement vs. stopped local-mode runner).
Remove the "pending" marker this card's id is placed under in docs/api.md and cli/context/runner.md.

Starts after #1294 is merged.