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.

src/main/database/schema.ts

@@ -3,6 +3,8 @@ import { createMigrationsV1V15 } from "./migrations/v1-v15";
 import { createMigrationsV16V25 } from "./migrations/v16-v25";
 import { createMigrationsV26V34 } from "./migrations/v26-v34";
 import { createMigrationsV35V37 } from "./migrations/v35-v37";
+import { createMigrationsV38V39 } from "./migrations/v38-v39";
+import { createMigrationsV40V40 } from "./migrations/v40-v40";
 import {
   columnExists,
   nowIso,
@@ -172,6 +174,8 @@ export function runMigrations(
     ...createMigrationsV16V25(deps),
     ...createMigrationsV26V34(deps),
     ...createMigrationsV35V37(deps),
+    ...createMigrationsV38V39(deps),
+    ...createMigrationsV40V40(deps),
   ];
 
   for (const migration of migrations) {