feat: assign issues to milestones on create/update (#22)

Add a `milestone` argument to `create_issue` and `update_issue` accepting
either a numeric milestone id or a title (resolved case-insensitively against
open and closed milestones, with a clear error for unknown titles). On
`update_issue`, `milestone: 0` clears the milestone. A BeforeValidator rejects
booleans so they are not silently coerced to an id.

Gitea Projects (Kanban boards) were investigated for #22 and are intentionally
left unsupported: Gitea 1.26.2 exposes no project endpoints in its REST API.
Documented this in api-reference.md and refreshed the (stale) write-mode tool
list to cover all 16 write tools.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/aegis_gitea_mcp/mcp_protocol.py

@@ -464,6 +464,10 @@ AVAILABLE_TOOLS: list[MCPTool] = [
                 "body": {"type": "string", "default": ""},
                 "labels": {"type": "array", "items": {"type": "string"}, "default": []},
                 "assignees": {"type": "array", "items": {"type": "string"}, "default": []},
+                "milestone": {
+                    "type": ["integer", "string"],
+                    "description": "Milestone id or title to assign the issue to",
+                },
             },
             "required": ["owner", "repo", "title"],
             "additionalProperties": False,
@@ -472,7 +476,7 @@ AVAILABLE_TOOLS: list[MCPTool] = [
     ),
     _tool(
         "update_issue",
-        "Update issue title/body/state (write-mode only).",
+        "Update issue title/body/state/milestone (write-mode only).",
         {
             "type": "object",
             "properties": {
@@ -482,6 +486,10 @@ AVAILABLE_TOOLS: list[MCPTool] = [
                 "title": {"type": "string"},
                 "body": {"type": "string"},
                 "state": {"type": "string", "enum": ["open", "closed"]},
+                "milestone": {
+                    "type": ["integer", "string"],
+                    "description": "Milestone id or title to assign; 0 clears the milestone",
+                },
             },
             "required": ["owner", "repo", "issue_number"],
             "additionalProperties": False,