feat(hub): add real read-only Activity timeline

[theDakshJaitly]

Summary

• add a real, authenticated, bounded read-only Activity API backed by Lane C canonical activity files and legacy decisions.jsonl
• surface exact canonical Activity counts on Home and preserve recorded actors while projecting current identity resolution
• replace the Activity placeholder with a date-grouped, filterable, paginated desktop workbench
• keep Catch Up, activity creation, Inbox, Relays, Workstreams, Graph/Wiki enrichment, and deep links deferred

Safety and boundaries

• production reads do not create/migrate local state, advance cursors, write events, or mutate Git/Graph/Wiki files
• canonical metadata and legacy cwd/trace/origin/raw content are omitted from the Hub projection
• fixtures remain development/test-only and are absent from production assets
• package-root exports and generated public declarations remain unchanged

Verification

• 746 root tests
• 57 Hub web tests
• 15 Playwright/axe/browser tests on Darwin
• root/Hub typechecks, evaluator suites, production build, packed-install smoke, package dry run, and declaration-leak checks

This is a draft stacked PR into feat/project-hub-foundation; it must not be merged into main directly.