feat(sessions): expose label in sessions.list and support label lookup in sessions_send

- Add `label` field to session entries and expose it in `sessions.list`
- Display label column in the web UI sessions table
- Support `label` parameter in `sessions_send` for lookup by label instead of sessionKey

- `sessions.patch`: Accept and store `label` field
- `sessions.list`: Return `label` in session entries
- `sessions_spawn`: Pass label through to registry and announce flow
- `sessions_send`: Accept optional `label` param, lookup session by label if sessionKey not provided
- `agent` method: Accept `label` and `spawnedBy` params (stored in session entry)

- Add `label` column to sessions table in web UI

- Changed session store writes to merge with existing entry (`{ ...existing, ...new }`)
  to preserve fields like `label` that might be set separately

We attempted to implement label persistence "properly" by passing the label
through the `agent` call and storing it during session initialization. However,
the auto-reply flow has multiple write points that overwrite the session entry,
and making all of them merge-aware proved unreliable.

The working solution patches the label in the `finally` block of
`runSubagentAnnounceFlow`, after all other session writes complete.
This is a workaround but robust - the patch happens at the very end,
just before potential cleanup.

A future refactor could make session writes consistently merge-based,
which would allow the cleaner approach of setting label at spawn time.

```typescript
// Spawn with label
sessions_spawn({ task: "...", label: "my-worker" })

// Later, find by label
sessions_send({ label: "my-worker", message: "continue..." })

// Or use sessions_list to see labels
sessions_list() // includes label field in response
```

src/agents/subagent-registry.ts

@@ -11,6 +11,7 @@ export type SubagentRunRecord = {
   requesterDisplayKey: string;
   task: string;
   cleanup: "delete" | "keep";
+  label?: string;
   createdAt: number;
   startedAt?: number;
   endedAt?: number;
@@ -83,6 +84,7 @@ function ensureListener() {
         ? (evt.data.endedAt as number)
         : Date.now();
     entry.endedAt = endedAt;
+    
     if (!beginSubagentAnnounce(evt.runId)) {
       if (entry.cleanup === "delete") {
         subagentRuns.delete(evt.runId);
@@ -101,6 +103,7 @@ function ensureListener() {
       waitForCompletion: false,
       startedAt: entry.startedAt,
       endedAt: entry.endedAt,
+      label: entry.label,
     });
     if (entry.cleanup === "delete") {
       subagentRuns.delete(evt.runId);
@@ -124,6 +127,7 @@ export function registerSubagentRun(params: {
   requesterDisplayKey: string;
   task: string;
   cleanup: "delete" | "keep";
+  label?: string;
 }) {
   const now = Date.now();
   const archiveAfterMs = resolveArchiveAfterMs();
@@ -136,6 +140,7 @@ export function registerSubagentRun(params: {
     requesterDisplayKey: params.requesterDisplayKey,
     task: params.task,
     cleanup: params.cleanup,
+    label: params.label,
     createdAt: now,
     startedAt: now,
     archiveAtMs,
@@ -175,6 +180,7 @@ async function probeImmediateCompletion(runId: string) {
       waitForCompletion: false,
       startedAt: entry.startedAt,
       endedAt: entry.endedAt,
+      label: entry.label,
     });
     if (entry.cleanup === "delete") {
       subagentRuns.delete(runId);