refactor(gitea-prs): retrofit to the ADR-0020 context contract

Description 709 -> 161 chars, body 683 -> 353 words, Gotchas 8 entries/56% of
body -> 2/24.9%. Clears the description FAIL and both Vale CompositionNote
errors.

Fixes the three stale claims recorded on issue #99, all re-verified against the
deployed gitea-mcp schema during review:

- The description no longer advertises 'reviewers' as an update capability.
  editPullRequestFn never reads reviewers or team_reviewers; only
  add_reviewers/remove_reviewers do.
- milestone is now marked honoured on "update" only, in the Gotcha, the body
  and the dispatch table's create row. On "create" the server discards it and
  omits the key from the response, so the drop is indistinguishable from never
  passing it -- and labels DOES apply on create, so labels landing is no
  evidence the milestone did. The old text told callers to resolve a milestone
  before any write, wasting the lookup on create.
- The superseded un-draft workaround is gone. "update" with draft:false and no
  title makes the server strip the prefix itself, including [WIP],
  case-insensitively -- carried by references/pull-requests.md, corrected in
  PR #106.

The 22-row tool/method table becomes a 5-row dispatch table; all 20 operations
it named remain reachable, including update_branch and the reviewer methods.

Refs #99

plugins/gitea/.apm/skills/gitea-prs/references/sources.md

@@ -21,7 +21,7 @@
 - **URL:** context7:/websites/gitea
 - **Description:** Official Gitea docs mirror on Context7 — branch protection rules, PR review/merge gating behavior, and automatic issue/PR cross-reference linking. Backfills the external/best-practice gap left by the original docs.gitea.com fetch timeout.
 - **Research doc:** plugins/gitea/docs/research/docs/gitea/sources.md
-- **Contributing files:** SKILL.md, references/merging.md
+- **Contributing files:** references/merging.md
 - **Status:** `extracted`
 
 ## context7-gitea-tea-cli