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/orchestration/run-dispatch.ts

@@ -71,6 +71,13 @@ export function createRunDispatch(deps: OrchestrationDependencies) {
     // Previously the worker claimed the lock only after it emitted `running`,
     // so two runs could race between enqueue and startup and both read the
     // same workspace mid-write. Main is the authoritative coordinator.
+    //
+    // On contention we mark the run `cancelled`, not `failed`. The Dashboard
+    // "Needs attention" card only surfaces failed/timed_out/interrupted runs
+    // (see src/renderer/components/dashboard/Dashboard.tsx), so the user is
+    // not alarmed by a transient backoff. The task row is rolled back to
+    // `todo` via the `finishRun` never-started branch, so heartbeat picks
+    // it up on the next cycle with no manual intervention.
     if (
       params.workspaceMode === "exclusive" &&
       params.workspace &&
@@ -78,9 +85,9 @@ export function createRunDispatch(deps: OrchestrationDependencies) {
     ) {
       db.finishRun({
         runId: params.runId,
-        status: "failed",
+        status: "cancelled",
         summary: null,
-        errorMessage: `Workspace ${params.workspace.id} is locked by another run.`,
+        errorMessage: `Workspace ${params.workspace.id} is in use by another run; will retry on the next heartbeat.`,
         exitCode: null,
         signal: null,
         model: null,