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>

src/aegis_gitea_mcp/tools/arguments.py

@@ -253,6 +253,70 @@ class RemoveLabelsArgs(RepositoryArgs):
     labels: list[str] = Field(..., min_length=1, max_length=20)
 
 
+class CreatePullRequestArgs(RepositoryArgs):
+    """Arguments for create_pull_request."""
+
+    title: str = Field(..., min_length=1, max_length=256)
+    head: GitRef = Field(..., min_length=1, max_length=200)
+    base: GitRef = Field(..., min_length=1, max_length=200)
+    body: str = Field(default="", max_length=20_000)
+
+
+class CreateReleaseArgs(RepositoryArgs):
+    """Arguments for create_release."""
+
+    tag_name: GitRef = Field(..., min_length=1, max_length=200)
+    name: str = Field(default="", max_length=256)
+    body: str = Field(default="", max_length=20_000)
+    draft: bool = Field(default=False)
+    prerelease: bool = Field(default=False)
+    target: str | None = Field(default=None, min_length=1, max_length=200)
+
+
+class EditReleaseArgs(RepositoryArgs):
+    """Arguments for edit_release."""
+
+    release_id: int = Field(..., ge=1)
+    name: str | None = Field(default=None, max_length=256)
+    body: str | None = Field(default=None, max_length=20_000)
+    draft: bool | None = Field(default=None)
+    prerelease: bool | None = Field(default=None)
+
+    @model_validator(mode="after")
+    def require_change(self) -> EditReleaseArgs:
+        """Require at least one mutable field in the update payload."""
+        if (
+            self.name is None
+            and self.body is None
+            and self.draft is None
+            and self.prerelease is None
+        ):
+            raise ValueError("At least one of name, body, draft, or prerelease must be provided")
+        return self
+
+
+class CreateBranchArgs(RepositoryArgs):
+    """Arguments for create_branch."""
+
+    new_branch_name: GitRef = Field(..., min_length=1, max_length=200)
+    old_branch_name: str | None = Field(default=None, min_length=1, max_length=200)
+
+
+class CreateMilestoneArgs(RepositoryArgs):
+    """Arguments for create_milestone."""
+
+    title: str = Field(..., min_length=1, max_length=256)
+    description: str = Field(default="", max_length=10_000)
+    due_on: str | None = Field(default=None, max_length=64)
+
+
+class EditIssueCommentArgs(RepositoryArgs):
+    """Arguments for edit_issue_comment."""
+
+    comment_id: int = Field(..., ge=1)
+    body: str = Field(..., min_length=1, max_length=10_000)
+
+
 def extract_repository(arguments: dict[str, object]) -> str | None:
     """Extract `owner/repo` from raw argument mapping.