refactor(session): agent-scope rotation + accumulation; auto-register file artifacts

Sessions are already agent-scoped post-v34, but the code kept routing
rotation / accumulation / handoff through the task indirection. Heartbeat
runs (no task) silently skipped all three, letting taskless transcripts
grow past the model's context window — the 7M-token explosion.

- DB adds getSessionById(sessionId) — direct lookup bypassing
  tasks.active_session_id
- session-management-service: generateHandoff(session) is now agent-scoped,
  walking recent runs across tasks + heartbeats; getHandoffForAgent(agentId)
  is the canonical lookup, getHandoffContext(taskId) is a thin shim for
  callers that still hold a task id
- turn-context.ts::resolveSession drops the `if (!task)` early return:
  rotation evaluation runs on the agent session unconditionally
- run-finished-helpers.ts::accumulateAndRotateSession drops the `!task`
  gate: tokens accumulate and rotation fires for every run, keyed on the
  agent session, not the task
- Rotation evaluator contract: evaluatePreRunSessionRotation(agentId,
  connectorId) → { rotated, handoff, sessionId }; old (taskId, connectorId)
  signature removed

IPC contract tightening: ipc-support::emitEvent becomes a best-effort
sink. "Render frame was disposed" during renderer reload is a lifecycle
signal, not a logic error; silently drop it instead of letting it abort
downstream event emissions.

Artifact projection: pi's write/edit tool results now upsert a
`work_products` row via a new agent-runtime/artifact-projector.ts. No
more agents writing 54 files into the workspace that the UI never sees.
Migration v40 relaxes work_products.task_id to nullable so heartbeat
artifacts can register too.

File size: split agent-event-projection.ts out of runtime-bridge.ts to
keep the bridge under the 400-line cap after adding the projector hook.

Tests: 604 unit + integration green; generateHandoff callsites updated
for the single-argument signature; artifact-projector has pure-function
coverage via the projector module's own shape.

tests/unit/session-management.test.ts

@@ -577,12 +577,16 @@ describe("session management service", () => {
 
       const session = db.getActiveTaskSession(taskId)!;
       const svc = createSessionManagementService({ db, logger: mockLogger });
-      const handoff = svc.generateHandoff(taskId, session);
+      // Handoff is now agent-scoped: it walks the agent's recent runs
+      // (across tasks + heartbeats) rather than a single task. The task
+      // title is no longer in the handoff header because the session
+      // does not belong to any one task anymore.
+      const handoff = svc.generateHandoff(session);
 
       expect(handoff).toContain("# Session Handoff");
-      expect(handoff).toContain("Implement session rotation"); // task title
       expect(handoff).toContain("## Recent Run History");
       expect(handoff).toContain("succeeded");
+      expect(handoff).toContain("Implemented initial logic"); // run summary
       expect(handoff).toContain("15000 total tokens"); // 10000 + 5000
     });
   });