Slice 7: expose git commands as MCP tools

Add git_fetch/git_pull/git_push/git_commit/git_discard_changes MCP tools as thin adapters over the service (actor=claude), completing 1.7 symmetry so Claude can run the same commands as the right-click menu. git_discard_changes is flagged destructive (confirm first, 1.4). Extended the MCP test with a git_commit round-trip; synced AGENT.md 8.1 tool list.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

AGENT.md

@@ -365,8 +365,9 @@ obeys the safety rules (§1.4).
   the tool handlers (§1.7). Expected tools (grow as features land):
   - Read: `list_repos`, `get_repo`, `get_active_project`, `get_activity`,
     `get_pending_switch`, `list_prs`.
-  - Act: `git_status/checkout/commit/push/pull/create_branch`,
-    `create_pr`, `merge_and_cleanup_pr`, `set_active_project`, `ack_switch`.
+  - Act: `git_fetch`, `git_pull`, `git_push`, `git_commit`,
+    `git_discard_changes`, `merge_and_cleanup_pr`, `set_active_project`,
+    `ack_switch`. (More — `git_checkout`, `create_branch`, `create_pr` — as they land.)
 - **A tool's result type must be a struct, never a bare slice/map/scalar.** The
   go-sdk infers each tool's `outputSchema` from its handler's result type, and MCP
   structured output must be a JSON **object** (`type: "object"`). A handler that