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>

docs/write-mode.md

@@ -13,14 +13,26 @@ Write mode introduces mutation risk (issue/PR changes, metadata updates). Risks
 
 ## Supported Write Tools
 
-- `create_issue`
-- `update_issue`
+- `create_issue` (optional `milestone` id or title)
+- `update_issue` (optional `milestone`; `0` clears it)
 - `create_issue_comment`
 - `create_pr_comment`
+- `edit_issue_comment`
 - `add_labels`
+- `remove_labels`
 - `assign_issue`
+- `create_label`
+- `update_label`
+- `create_pull_request`
+- `create_release`
+- `edit_release`
+- `create_branch`
+- `create_milestone`
 
-Not supported (explicitly forbidden): merge actions, branch deletion, force push.
+Not supported (explicitly forbidden): merge actions, branch/label/release deletion,
+force push, repo/admin management, and repository content writes (file create/edit,
+commits). Gitea Projects (Kanban boards) are unsupported because the Gitea REST API
+exposes no project endpoints.
 
 ## Enablement Steps