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

Description 1012 -> 347 chars, body 582 -> 170 words, Gotchas 3 entries -> 1 at
22.9% of body. Clears the description FAIL and all four Vale CompositionNote
errors -- the last carriers in the corpus, so that rule now fires nowhere.

Cut the 'human-facing entry point and router' architecture note, the /gitea
migration history and the six-skill composition list; all were already in the
README or the routing table.

Split three mutually exclusive flows into a dispatch table keyed on invocation
shape, each branch self-contained in references/: status-checkin.md,
number-resolution.md, skill-index.md. Report stays in the body as the gate
common to every branch; each branch's own format moved to its file. The old
Step 1-4 numbering presented three alternatives as a sequence.

The description grew from an intermediate 283 chars on purpose: that draft had
dropped flow 3's trigger entirely, leaving the domain-routing index -- a third
of the skill -- reachable only through a tail clause whose grammatical subject
was the request rather than the skill. Both gates were green over that.

Boundary clauses are one arrow per target, so both resolve (#107: the resolver
extracts only the first target per clause and reports 1 of 1 on a clause naming
two). The local-git exclusion keeps its wording but drops the route to
git-workflow, which would not resolve in a gitea-only install.

Known residual: the dispatch conditions are stated twice, as a table and as
literal conditionals. That is #109 -- body-discipline.md mandates the literal
form while the ADR's cited exemplar, apm-workflow, uses a bare table plus one
summary line. Fixing it here would settle that contradiction in a skill rather
than in the spec, so it rides with #109.

Refs #99, #107, #109

plugins/gitea/.apm/skills/gitea-workflow/references/skill-index.md

@@ -0,0 +1,22 @@
+---
+topic: skill-index
+source_keys:
+  - gitea-mcp-repo
+---
+
+# Domain-skill index
+
+The request names a capability but not obviously which skill owns it. Find the owner here, then invoke it:
+
+| Skill | Covers |
+|---|---|
+| `gitea-issues` | List/read/create/update issues, comments, search across issues and PRs. Composes `gitea-labels-milestones` for label/milestone resolution. |
+| `gitea-labels-milestones` | Label and milestone CRUD, label inference from conversation context, resolving names/titles to the numeric IDs writes require. Cross-cutting — used by both `gitea-issues` and `gitea-prs`. |
+| `gitea-prs` | List/read/create/update/merge PRs, code reviews. Composes `gitea-labels-milestones` the same way `gitea-issues` does. |
+| `gitea-branches` | Branch list/create/delete, plus commit history (list commits, get a single commit by SHA). |
+| `gitea-files` | Read/write/delete individual files, list a directory, walk the full repo tree. |
+| `gitea-releases` | Release and tag CRUD — draft/prerelease flags, release notes, semver tags. |
+
+A request that already names its own owner (e.g. "create a milestone" → `gitea-labels-milestones`, "read this file from the repo" → `gitea-files`) never needed this index — invoke that skill directly rather than routing through here.
+
+Then report per `SKILL.md`'s Report section: name the skill and hand off — don't duplicate its output format, let it report.