feat: record agent sessions and operator halts in supervised rounds (P2)

[TKontu]

This is phase P2 of the autonomous Reconcile-loop plan. It is stacked on #7: after #7 merges, retarget this PR to autonomous-reconcile-loop.

New version 2 ledger commands in scripts/round.py:

• session start ROUND --role planner|executor|reviewer|merger …
  • It is a write-ahead record of each agent launch: runner, session UUID, worktree, head, deadline. It prints {"session": "S<n>"}.
  • It refuses:
    • a duplicate UUID
    • a second live session on an assignment
    • a second live planner
    • any start while a halt is uncleared
  • Role rules:
    • an executor needs assigned, or a needs-fix review (a repair)
    • a reviewer needs the latest head with no review yet
    • a merger needs a ready review of that head
• session ack --pid moves a session from launching to running. session end --state exited|timed-out|killed|lost records the exit code, cost and the imported record (the record path must exist).
• review --session S is an alternative to --reviewer. It accepts only a finished reviewer session for the same assignment and head.
• halt and clear-halt keep a durable record with code, detail, action needed and an optional inventory JSON. Clearing needs evidence.
• Guards:
  • Cancellation refuses while the assignment has a live session.
  • A merged resolution tolerates only its live merger session.
  • Supervised closure refuses live sessions and uncleared halts.
• Version 1 rounds refuse all of these commands.

SUPERVISION.md documents sessions and halts.

Validation:

• RED: 6 of the 7 new tests failed before implementation. The version 1 refusal guard passed already.
• GREEN: python3 -m unittest discover -s tests passes (41 tests), also with ResourceWarnings promoted to errors. git diff --check is clean.

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