feat: add PR/release/branch/milestone/comment write tools

Adds six opt-in write tools (write-mode + policy + per-user permission still
enforced; no destructive or admin actions):

- create_pull_request (POST /pulls)
- create_release / edit_release (POST/PATCH /releases)
- create_branch (POST /branches; create only, no deletion)
- create_milestone (POST /milestones)
- edit_issue_comment (PATCH /issues/comments/{id})

Each: arg schema (extra=forbid, GitRef on branch/ref-like fields), Gitea client
method with url-encoded path segments, handler that surfaces auth errors, MCP
registration (write_operation=True), server wiring, docs, and success tests.

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

docs/api-reference.md

@@ -70,6 +70,15 @@ Scope requirements:
 - `assign_issue` (`owner`, `repo`, `issue_number`, `assignees`)
 - `create_label` (`owner`, `repo`, `name`, `color` hex e.g. `#00aabb`, optional `description`, `exclusive`)
 - `update_label` (`owner`, `repo`, `name`, one or more of `new_name`, `color`, `description`)
+- `create_pull_request` (`owner`, `repo`, `title`, `head`, `base`, optional `body`)
+- `create_release` (`owner`, `repo`, `tag_name`, optional `name`, `body`, `draft`, `prerelease`, `target`)
+- `edit_release` (`owner`, `repo`, `release_id`, one or more of `name`, `body`, `draft`, `prerelease`)
+- `create_branch` (`owner`, `repo`, `new_branch_name`, optional `old_branch_name`)
+- `create_milestone` (`owner`, `repo`, `title`, optional `description`, `due_on`)
+- `edit_issue_comment` (`owner`, `repo`, `comment_id`, `body`)
+
+Not supported by design: merge, branch/label/release deletion, force push, repo/admin
+management.
 
 Note: `create_issue`, `add_labels`, and `remove_labels` accept label **names**; the
 server resolves them to Gitea label ids and returns a clear error for unknown labels.