feat(wiki): add wiki skill for durable, project-neutral wiki articles

Standalone utility skill (manual /wiki, not a pipeline phase). Fixes the house rules for repository-wiki articles: durable project-neutral knowledge only (no implementation state — that belongs in docs/), every load-bearing fact backed by a validated external source, claims marked law/convention/corrected, an overview page plus cross-linked detail pages each with a References section, written in English. Documents the wiki mechanics (own <repo>.wiki.git, slugs, edit paths, special files) and the wiki-vs-docs/ decision rule.

README: list `wiki` among the on-demand utility skills, kept out of the pipeline table.

README.md

@@ -30,9 +30,11 @@ The pipeline skills, each with the agents it primarily dispatches:
 | `docwriter` | API surface stable across N cycles | rustdoc / docstring sweep | Optional |
 | `boss` | User types `/boss` | autonomous-orchestrator session — dispatches the other skills until done-state or bounce-back; can optionally sign specs in the user's place (opt-in, see below) | User-invoked, never auto-dispatched |
 
-Two further **utility skills** are invoked on demand rather than as
-pipeline phases: `issue` (file or update a tracker item) and `glossary`
-(build or maintain the project glossary — see `docs/glossary-convention.md`).
+Three further **utility skills** are invoked on demand rather than as
+pipeline phases: `issue` (file or update a tracker item), `glossary`
+(build or maintain the project glossary — see `docs/glossary-convention.md`),
+and `wiki` (write or edit a repository-wiki article — durable, project-neutral,
+sourced knowledge, deliberately kept out of `docs/` and the pipeline).
 
 One **conversational skill** stands outside the pipeline entirely:
 `pseudo` (typed `/pseudo`) switches replies into commented,