feat(ui): hide observer-sessions project from UI lists

Observer sessions (internal SDK-driven worker queries) run under a
synthetic project name 'observer-sessions' to keep them out of
claude --resume. They were still surfacing in the viewer project
picker and unfiltered observation/summary/prompt feeds.

Filter them out at every UI-facing query:
- SessionStore.getAllProjects and getProjectCatalog
- timeline/queries.ts getAllProjects
- PaginationHelper observations/summaries/prompts when no project is selected

When a caller explicitly requests project='observer-sessions',
results are still returned (not a hard ban, just hidden by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/shared/paths.ts

@@ -75,6 +75,10 @@ export const VECTOR_DB_DIR = join(DATA_DIR, 'vector-db');
 // Sessions here won't appear in user's `claude --resume` for their actual projects
 export const OBSERVER_SESSIONS_DIR = join(DATA_DIR, 'observer-sessions');
 
+// Project name assigned to observer sessions (basename of OBSERVER_SESSIONS_DIR).
+// UI queries filter this out so internal worker sessions don't pollute project lists.
+export const OBSERVER_SESSIONS_PROJECT = 'observer-sessions';
+
 // Claude integration paths
 export const CLAUDE_SETTINGS_PATH = join(CLAUDE_CONFIG_DIR, 'settings.json');
 export const CLAUDE_COMMANDS_DIR = join(CLAUDE_CONFIG_DIR, 'commands');