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]>

docs/webui-local-dev.md

@@ -80,6 +80,8 @@ status, onboarding checklist state, and the fail-closed error payloads (#635).
 | `/sessions` | Runtime and session view (#641) — health + inventory sessions/namespaces/worktrees |
 | `/api/sessions` | JSON export for the runtime/session view |
 | `/api/v1/sessions` | Versioned alias of `/api/sessions` |
+| `/gitea` | Gitea issue↔PR linkage console (#645) — both directions, with the evidence for each edge |
+| `/api/v1/gitea/linkage` | JSON linkage export; `502` when the read could not be answered |
 | `/inventory` | Phase 1 shell stub — unified inventory (backed by #636) |
 | `/timeline` | Phase 1 shell stub — workflow event timeline |
 | `/policy` | Phase 1 shell stub — capability/role policy placeholder |
@@ -327,6 +329,68 @@ Honesty rules specific to this view:
   The write-time redactor is a narrow denylist and is not relied on. The field
   itself is kept — it is the `#630` evidence naming which daemon was killed.
 
+## Gitea issue/PR linkage (#645)
+
+`/gitea` is the Phase 3 read-only linkage console: which PR carries which issue,
+which issues are claimed by more than one PR, and what the latest Canonical
+Thread Handoff on a thread said. Gitea remains the source of truth — this
+surface reads it and never writes to it. There is no issue/PR editor, no review,
+and no merge control.
+
+Query parameters (all optional):
+
+| Parameter | Meaning |
+|-----------|---------|
+| `project` | Registry project id to scope the read (default: first registry entry) |
+| `state` | `open` (default) or `all`; `all` widens the window to merged/closed items, where a landed edge lives |
+| `issue=N` / `pr=N` | Focus one thread and load *its* latest canonical handoff |
+
+`GET /api/v1/gitea/linkage` returns the same model as JSON
+(`schema_version: 1`). It answers `502` when the read could not be answered, so
+an automated consumer cannot mistake a fail-closed payload for "no links exist".
+The HTML page always answers `200` and renders the reason instead — an operator
+view must show why a read failed rather than withhold the page.
+
+### How an edge is found
+
+Each edge carries the evidence that produced it, strongest first:
+
+| Evidence | Meaning |
+|----------|---------|
+| `closes_keyword` | The PR title or body declares `closes/fixes/resolves #N`. Gitea itself acts on this keyword. |
+| `branch_marker` | The PR head branch carries the canonical `(fix\|feat\|docs\|chore)/issue-N-…` marker minted by the issue lock. |
+| `body_reference` | The PR body mentions `#N` with no closing keyword. A mention is not a claim to close. |
+
+Only closing and branch-marker edges populate the **issue → PR** direction: a
+bare mention is a cross-link, and counting it as ownership would invent
+contested issues out of ordinary references. The mention stays visible on the
+**PR → issue** side, labelled as such. A PR whose two strongest edges tie is
+flagged `ambiguous`; an issue claimed by two PRs is flagged `contested`.
+
+### Honesty rules specific to this view
+
+* **A partial read never reads as an absence.** Linkage is a claim about the
+  loaded window only. When pagination did not complete, every empty edge cell
+  renders `none found (partial inventory)` rather than `none`, and the JSON
+  carries `inventory_complete: false` plus per-row `links_authoritative: false`.
+* **A failed read renders no table at all.** Missing credentials, an unknown
+  project, or a fetch error produce `ok: false` with a reason. An empty linkage
+  table would assert that no issue is linked to any PR, which such a read is not
+  in a position to claim.
+* **Handoffs are loaded, never assumed.** CTH comments are thread-scoped, so
+  only the focused issue or PR has its comments fetched. Every other row reports
+  `not_loaded` with the reason; a thread whose comments *were* loaded and carried
+  no CTH says exactly that. A comment-source failure degrades the handoff alone —
+  the linkage tables still render.
+* **Unrecognised handoff headings are reported, not republished.** A `## CTH:`
+  heading outside `CTH_TYPES` renders as `unrecognized`.
+* **Redaction precedes display.** Titles, labels, handoff fields, and error
+  reasons pass through `webui.console_redaction` before serialization, and the
+  page HTML-escapes everything it renders.
+* **Deep links are opt-in.** A link out to the Gitea web UI appears only when
+  `GITEA_MCP_REVEAL_ENDPOINTS=1` is set server-side, matching how the MCP tools
+  gate URL exposure. Item numbers stay usable without it.
+
 ## System-health dashboard (#639)
 
 `/system-health` renders the same snapshot the `/api/v1/system/health` API