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/api-reference.md

@@ -74,8 +74,8 @@ Scope requirements:
 
 ## Write Tools (Write Mode Required)
 
-- `create_issue` (`owner`, `repo`, `title`, optional `body`, `labels`, `assignees`)
-- `update_issue` (`owner`, `repo`, `issue_number`, one or more of `title`, `body`, `state`)
+- `create_issue` (`owner`, `repo`, `title`, optional `body`, `labels`, `assignees`, `milestone`)
+- `update_issue` (`owner`, `repo`, `issue_number`, one or more of `title`, `body`, `state`, `milestone`)
 - `create_issue_comment` (`owner`, `repo`, `issue_number`, `body`)
 - `create_pr_comment` (`owner`, `repo`, `pull_number`, `body`)
 - `add_labels` (`owner`, `repo`, `issue_number`, `labels` by name)
@@ -96,6 +96,12 @@ 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.
 
+Note: the `milestone` argument on `create_issue`/`update_issue` accepts either a numeric
+milestone **id** or a milestone **title** (resolved case-insensitively against open and
+closed milestones; unknown titles return a clear error). On `update_issue`, `milestone: 0`
+clears the issue's milestone. Gitea Projects (Kanban boards) are intentionally unsupported:
+the Gitea REST API exposes no project endpoints.
+
 ## Validation and Limits
 
 - All tool argument schemas reject unknown fields.