feat(webui): Gitea issue and PR linkage console (Closes #645)

Add a Phase 3 read-only console that resolves issue↔PR linkage with
evidence (closes keyword, branch marker, body mention), surfaces the
latest canonical handoff for a focused thread, and deep-links to Gitea
only under the admin reveal opt-in.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

webui/nav.py

@@ -6,7 +6,8 @@ destination is a GET view or a Phase 1 placeholder. No mutation links.
 
 Nav groups follow the #631 Phase 1 information architecture: Health, Traffic,
 Runtime/Sessions, Projects, Inventory, Timeline, Policy (placeholder), and
-Insights (placeholder). Later-phase surfaces are declared as ``stub`` items and
+Insights (placeholder), joined by the Phase 3 Gitea linkage group (#645).
+Later-phase surfaces are declared as ``stub`` items and
 backed by ``STUB_PAGES`` so their nav links resolve to a graceful placeholder
 instead of a 404.
 """
@@ -60,6 +61,9 @@ NAV_GROUPS: tuple[NavGroup, ...] = (
     NavGroup("Timeline", (
         NavItem("/timeline", "Timeline", "stub"),
     )),
+    NavGroup("Gitea", (
+        NavItem("/gitea", "Issue/PR linkage"),
+    )),
     NavGroup("Policy", (
         NavItem("/policy", "Policy", "stub"),
         NavItem("/prompts", "Prompts"),