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

Description 688 -> 282 chars, body 435 -> 293 words, Gotchas 5 entries/54% of
body -> 3/23.5%. Clears the description FAIL and the Gotchas suggestion.

Cut the second trigger register (six re-quoted user phrasings) and the capability
enumeration; both moved to a new Boundaries section in the skill's own README.
Kept all three boundary clauses -- git-branches, git-history and gitea-prs each
defend a real activation steal, and gitea-prs is now the only guard on the
branch/PR collision in either direction since gitea-prs's own retrofit narrowed
its boundary to issues. Written as one arrow per target: the resolver extracts
only the first name per arrow clause, so conjoined targets go unchecked.

Two Gotchas deleted -- one paraphrased the step below it (its non-obvious half,
the get_me/list_my_repos token-scope block, was folded into that step), the other
is carried in full by references/branches.md:40-52.

Repoint two reference pointers the rename broke: branches.md and commits.md named
Gotchas by titles this retrofit changed. Now named by stable descriptors.

Refs #99

plugins/gitea/.apm/skills/gitea-branches/references/branches.md

@@ -65,7 +65,7 @@ A branch name collision returns `409 Conflict`.
 delete_branch owner: <owner> repo: <repo> branch: <name>
 ```
 
-Before calling this, see the hard-refusal Gotcha in SKILL.md. If the target branch's name isn't
+Before calling this, see the `delete_branch` Gotcha in SKILL.md. If the target branch's name isn't
 obviously a scratch/feature branch, call `list_branches` first and check `protected` on the
 matching entry — name-matching `main`/`master` alone isn't authoritative, since a repo can protect
 a differently-named default branch. Confirm explicitly with the user before deleting anything