feat(sessions): add Reload action to refresh session from disk

Sessiond caches the in-memory SessionManager and never re-reads the
session file. When the same session is also being edited by another
process (e.g. the pi CLI), new entries on disk are invisible to the
web UI \u2014 the tail of the conversation gets cut.

Add a manual Reload action in the session three-dot menu:

- Server: PiSessionService.reload(sessionId) closes the active
  session and re-opens it from disk, then publishes a fresh status.
  Exposed as POST /api/.../sessions/:sessionId/reload.
- Client: api.reloadSession, SessionController.reloadSession which
  discards the cached transcript and re-runs selectSession so the
  history page is re-fetched.
- ChatTranscriptStore gains discard(sessionId) and the history
  cache adapter gains optional remove(sessionId).
- SessionList shows a Reload entry for non-archived, non-cached
  sessions; plumbed through AppNavigationPanel and PiWebApp.

Note: pi-web-sessiond.service must be restarted manually after this
change since the session daemon code path is affected.

src/server/sessions/piSessionService.ts

@@ -560,6 +560,13 @@ export class PiSessionService {
     await this.archiveStore.deleteArchived(record.sessionId);
   }
 
+  async reload(ref: PiSessionLookup): Promise<void> {
+    const active = await this.getActive(ref);
+    await this.closeActive(active.runtime.session.sessionId);
+    const reopened = await this.getActive(ref);
+    this.publishStatus(reopened.runtime.session);
+  }
+
   async detachParent(ref: PiSessionLookup): Promise<void> {
     const session = await this.getOrOpen(ref);
     const sessionFile = session.sessionFile;

src/server/sessions/sessionRoutes.ts

@@ -230,6 +230,15 @@ export function registerSessionRoutes(app: FastifyInstance, sessions: PiSessionS
     }
   });
 
+  app.post<{ Params: { sessionId: string }; Body: { cwd?: unknown } | undefined }>(`${prefix}/sessions/:sessionId/reload`, async (request, reply) => {
+    try {
+      await sessions.reload(sessionLookupFromBody(request.params.sessionId, optionalRecord(request.body)));
+      return { reloaded: true };
+    } catch (error) {
+      return reply.code(mutationErrorStatus(error)).send({ error: errorMessage(error) });
+    }
+  });
+
   app.post<{ Params: { sessionId: string }; Body: { cwd?: unknown } | undefined }>(`${prefix}/sessions/:sessionId/detach-parent`, async (request, reply) => {
     try {
       await sessions.detachParent(sessionLookupFromBody(request.params.sessionId, optionalRecord(request.body)));