Feat: transcript retention (#123)

* feat(retention): prune old agent transcripts + panel-tunable setting

Agents write a {session-id}.jsonl per spawn under ~/.claude/projects; nothing
ever deleted them, so the operator's bind-mounted ~/.claude grew without bound.

Add a throttled orchestrator sweep that prunes agent-owned transcripts (the
shared -app dir + per-workspace dirs) older than a retention window — and ONLY
agent-owned dirs, never the operator's own Claude sessions (proven by the
temp-dir selection tests). The window is panel-tunable: a new system_settings
key-value table (migration 027) holds transcript_retention_days, read via
SettingsService with the roboco.config default (14d) as the fallback, exposed
through GET/PUT /api/settings. Panel wiring follows.

* feat(panel): add a panel-tunable transcript retention control

Wire the settings page to the /api/settings backend: a settings API client and
a self-contained Transcript Retention card (React Query) that loads
transcript_retention_days and saves it back, with client-side validation. The
existing settings controls are unchanged.

---------

Co-authored-by: Renn F <rennf93@users.noreply.github.com>

panel/src/lib/api/index.ts

@@ -13,3 +13,4 @@ export { gitApi } from "./git";
 export { a2aApi } from "./a2a";
 export { streamApi } from "./stream";
 export { groupsApi } from "./groups";
+export { settingsApi } from "./settings";