fix(runtime): recognize client identity environment and refresh worker registrations

Two defects left behind by #948 made sanctioned multi-client operation
impossible. They are inseparable: fixing either alone still leaves the
multi-client canary unable to run.

1. Client-identity environment keys were not recognized.

   gitea_mcp_server reads GITEA_MCP_CLIENT, GITEA_MCP_CLIENT_INSTANCE and
   GITEA_MCP_CLIENT_SESSION as the authoritative client-identity inputs for
   worker registration, but none of the three appeared in
   RECOGNIZED_GITEA_ENV_KEYS or matched a recognized prefix. The runtime
   diagnostic scans every peer mcp_server.py process environment and classifies
   any unlisted GITEA_* key as an unsupported override, which is raised as a
   runtime blocker, so gitea_resolve_task_capability returned
   blocker_kind=runtime_reconnect_required with stop_required=true. Because that
   resolver is the mandatory preflight for every author, reviewer and merger
   mutation, setting the very variable #948 requires closed the mutation gate
   for the whole fleet, and reconnecting could not clear it: the variable is
   re-exported from the client's server definition on every launch.

   The three keys are now named individually in the recognized-key set. No
   prefix is added, so an unrecognized GITEA_* override is still refused
   exactly as before.

   A related inconsistency in the same path is also fixed. The diagnostic
   reasons are raised as one RuntimeError, but the preflight re-raise
   recognized only "stale-runtime:", so an "unsupported-env:" reason was
   silently swallowed there while still failing the resolver. Both reason
   families now live in RUNTIME_DIAGNOSTIC_HARD_PREFIXES beside the function
   that produces them, and both propagate identically. This only widens what is
   refused, never what is permitted.

2. WorkerRegistry.heartbeat() had no production caller.

   #948 delivered heartbeat() but only tests called it. The single production
   writer registers once per process behind an attempted-once flag, and
   register() stamps the same timestamp into both started_at and
   last_heartbeat_at. Nothing advanced it afterwards: no lifespan hook, no
   background task, no atexit handler in a process that blocks in mcp.run().
   Since liveness is age against heartbeat_ttl_seconds, that TTL was not a
   liveness window at all but a hard cap on how long any client could stay
   attached; at 900 seconds a healthy, connected, client-managed process became
   session_ownership=unowned with blocker_kind=session_attachment_missing.

   WorkerHeartbeatSupervisor in mcp_worker_identity is the missing caller,
   started from _active_worker_identity() at the moment register() succeeds,
   because that is the only point where identity and fencing_epoch are both
   known. It is a daemon thread rather than an asyncio task or a request-driven
   refresh because renewal must survive an idle session, and because the
   registry performs blocking BEGIN IMMEDIATE sqlite writes that must not run on
   the server's event loop. daemon=True is deliberate: a hard kill takes the
   thread with it, so a dead worker still goes stale on the normal TTL.

   heartbeat_interval_for() returns one third of the TTL, hard-capped at one
   half, so two consecutive beats can be lost without the row expiring and no
   override can produce an interval that outlives the registration it renews.
   heartbeat() gains optional keyword-only expectations (session, generation,
   client name, pid); each supplied one must match the recorded row or the
   renewal is refused with the existing BLOCKER_FENCED literal rather than a new
   blocker_kind, since consumers switch on that value. Omitting them preserves
   the pre-existing behavior exactly. Client names are compared normalized, so
   several namespaces of one application stay one client while separate
   applications stay distinct.

   A terminal refusal stops the supervisor permanently and records why, so a
   fenced session can never beat its way back into ownership. A transient
   failure is counted and beating continues. An atexit hook stops it on orderly
   shutdown. status() is surfaced read-only as worker_heartbeat on
   gitea_get_runtime_context so a stopped heartbeat is diagnosable before the
   TTL turns it into session_attachment_missing; it grants nothing.

   claim_generation() still has no production caller. It bumps fencing_epoch,
   which would fence the supervisor's cached epoch, and the strict refusal is
   left in place deliberately: auto-re-adopting a bumped epoch would defeat
   fencing.

No lock or lease TTL is changed, including the author issue-lock TTL, and no
mutation refused today becomes permitted.

Tests: tests/test_issue_975_client_identity_heartbeat.py adds 40 focused tests
covering all 13 acceptance criteria. Every TTL assertion uses an injected
clock; no test waits for a real TTL. The thread-loop tests use a
millisecond-scale interval with bounded polling.

Focused: 40 passed, 15 subtests passed.
Full suite from inside the branches worktree: 28 failed, 6105 passed, 6 skipped,
1105 subtests — an identical failure set to the 324a0c8a baseline measured in a
sibling branches worktree (28 failed, 6065 passed, 1090 subtests). Zero new
failures; the delta is exactly the added tests.

Closes #975

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

gitea_config.py

@@ -1193,6 +1193,16 @@ RECOGNIZED_GITEA_ENV_KEYS = frozenset({
     "GITEA_IRRECOVERABLE_HMAC_SECRET",
     "GITEA_FORCE_MCP_RUNTIME_CHECK",
     "GITEA_FORCE_CLIENT_MANAGED",
+    # #975: the client-identity inputs the server actually consumes at startup
+    # (CLIENT_NAME_ENV / CLIENT_INSTANCE_ENV / CLIENT_SESSION_ENV in
+    # gitea_mcp_server). Production read them while this allowlist omitted them,
+    # so the peer-env scan classified them as unsupported overrides and the
+    # capability resolver refused every mutation fleet-wide. Named individually
+    # on purpose: no prefix is added, so an unrecognised GITEA_* override is
+    # still refused exactly as it was before.
+    "GITEA_MCP_CLIENT",
+    "GITEA_MCP_CLIENT_INSTANCE",
+    "GITEA_MCP_CLIENT_SESSION",
 })
 
 RECOGNIZED_GITEA_ENV_PREFIXES = (