feat(git): merge the PR when a task is accepted

Closes 148/149 — the accept gate now lands code instead of just moving a row.

Driver gains createPullRequest / getPullRequest / mergePullRequest, verified
against the real Gitea API shapes rather than guessed: MergePullRequestOption
requires a capitalised `Do`, and supports `head_commit_id`.

That `head_commit_id` is the point. A human approved a SPECIFIC commit; if
someone pushed after that, merging anyway lands code nobody reviewed. So the
reviewed sha travels with the request and is checked twice — once by us for a
clear MergeStaleError naming both commits, and again by the forge, which closes
the race between our read and the merge. When it trips, the message says what
happened and what to do: re-review the current head, then accept again.

Accept merges BEFORE it transitions. If the forge refuses — conflicts, failing
checks, a moved branch — the task stays in awaiting_review and no event is
written. Reporting `merged` while main does not have the code is the single
worst thing this gate could do, and there is a test asserting the transition
never happens on a failed merge.

Failure modes are answers, not stack traces: no PR attached is approval-only and
succeeds; a repo with no forge connection explains that mcpctl cannot merge it;
an already-merged PR is treated as success so a retry after a timeout is safe.

owner/repo comes from `sourceFullName` when discovery recorded it, falling back
to parsing https, ssh:// and scp-style clone URLs, and returning null rather
than guessing when it cannot tell.

The merger is injected and optional, so an mcpd without a git provider still
runs and accept degrades to approval-only — which is also what the help text
now says, restored after the earlier commit that removed a claim we could not
back up.

Tests: 7 driver PR tests (Do field, head pinning, stale refusal, idempotent
re-merge, 405 → refusal), 10 merger tests incl. URL parsing, 7 accept-gate tests.

Local: build clean, workspace 2580 passed (+23), lint unchanged at 794 errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4wNHWf7xSwnZCWpJcyv9p

.taskmaster/tasks/tasks.json

@@ -3031,9 +3031,9 @@
         "dependencies": [
           "132"
         ],
-        "status": "pending",
+        "status": "done",
         "subtasks": [],
-        "updatedAt": "2026-08-08T08:55:58.627Z"
+        "updatedAt": "2026-08-14T23:05:31.782Z"
       },
       {
         "id": "149",
@@ -3046,9 +3046,9 @@
           "130",
           "148"
         ],
-        "status": "pending",
+        "status": "done",
         "subtasks": [],
-        "updatedAt": "2026-08-08T08:55:58.627Z"
+        "updatedAt": "2026-08-14T23:05:31.792Z"
       },
       {
         "id": "150",
@@ -3461,9 +3461,9 @@
     ],
     "metadata": {
       "version": "1.0.0",
-      "lastModified": "2026-08-10T11:11:23.912Z",
+      "lastModified": "2026-08-14T23:05:31.792Z",
       "taskCount": 177,
-      "completedCount": 134,
+      "completedCount": 136,
       "tags": [
         "master"
       ]