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/gateway/session-utils.ts

@@ -34,6 +34,7 @@ export type GatewaySessionsDefaults = {
 export type GatewaySessionRow = {
   key: string;
   kind: "direct" | "group" | "global" | "unknown";
+  label?: string;
   displayName?: string;
   provider?: string;
   subject?: string;
@@ -485,6 +486,7 @@ export function listSessionsFromStore(params: {
       return {
         key,
         kind: classifySessionKey(key, entry),
+        label: entry?.label,
         displayName,
         provider,
         subject,