* feat(subagents): add delegations ledger field + reducer to ThreadState
* feat(subagents): pure helpers to derive + format the delegation ledger
* feat(subagents): DelegationLedgerMiddleware records + injects the ledger
* feat(subagents): register DelegationLedgerMiddleware for lead when subagents enabled + docs
* add runtime log
* chore(subagents): make delegation-ledger injection log production-ready
* test(subagents): make delegation-ledger registration tests config-free; refresh ultra replay golden for delegations channel
* refactor(subagents): derive TERMINAL_STATUSES from SUBAGENT_STATUS_VALUES + pin it
Make thread_state's TERMINAL_STATUSES a frozenset over the status contract's
SUBAGENT_STATUS_VALUES instead of a hardcoded literal, so the terminal-status
set can never drift from the contract. Add a pinning test asserting the
derivation and that the non-terminal "in_progress" stays excluded.
Addresses PR #3877 review.
---------
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
