sx-gitea Phase 6: activity — feed timelines, dashboard, durable notifications (TDD, 520/520)

lib/gitea/activity.sx: every forge action lands as a feed activity in an
append-only persist log stream. Instrumentation is done IN the runtime —
repo-create!/issue-create!/issue-comment!/pr-create!/pr-review!/pr-merge!
are redefined around their originals, so SX callers and web handlers emit
activity with zero call-site edits (failed mutations emit nothing).

Timelines are lib/feed (APL) queries: global/repo/user, newest-first,
visibility follows repo access (private-repo activity invisible to
non-readers). Follows (user: or repo: targets) drive a dashboard of
followed actors/repos minus one's own actions.

Notifications ride lib/events durable delivery: activities after a
cursor expand to (id recipient body) messages (comment -> author+
participants, review/merge -> PR author, open-issue -> assignees, never
the actor), ev/deliver-messages runs the at-least-once digest flow, and
delivered messages file into per-user kv inboxes; the cursor advance
makes reruns no-ops.

Web: /activity + /:owner/:name/activity pages, user-activity/dashboard/
follow/notifications/notify-run JSON API. gitea/all-routes now hoists
every /api/* route ahead of the wildcard /:owner/:name patterns so later
packs can add API endpoints without being shadowed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/gitea/scoreboard.md

@@ -9,4 +9,5 @@ _Generated by `lib/gitea/conformance.sh`_
 | wire | 78 | 0 | 78 |
 | issues | 88 | 0 | 88 |
 | pr | 100 | 0 | 100 |
-| **Total** | **460** | **0** | **460** |
+| activity | 60 | 0 | 60 |
+| **Total** | **520** | **0** | **520** |