refactor(git): normalize rtk-prefix usage, add metadata.version

Two bundled fixes across the same nine skills, since both touch the
same files.

Issue #113: skill prose used rtk git and bare git inconsistently for
the same operations, with no stated rule for which applied where.
Executable instructed commands (a dispatch-table "Run" cell, a fenced
code-block procedure, an imperative step) now consistently use rtk
git; illustrative or referential mentions -- naming a flag's behavior,
quoting a doc heading, warning against an anti-pattern -- stay bare
git. Documented in the new plugins/git/README.md, scoped to this
plugin only: gitea-* skills talk to the server over MCP tools and
carry no git/rtk mentions at all.

Also the git-plugin slice of #127: metadata.version added to the
eight skills that lacked it. git-commits already had one and is
untouched.

Fixes: #113
Fixes: #127
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EeH8SCbcrCAQrtymkNuhKP

plugins/git/.apm/skills/pc-author/references/modify-config.md

@@ -17,7 +17,7 @@ Read the existing `.pre-commit-config.yaml` before editing. Note any stale `rev`
 1. Run a shallow extension scan, so the addition is judged against the languages actually present:
 
    ```bash
-   git ls-files | grep -oE '\.[a-z]+$' | sort | uniq -c | sort -rn
+   rtk git ls-files | grep -oE '\.[a-z]+$' | sort | uniq -c | sort -rn
    ```
 
 2. Read `references/hooks-by-language.md` for the correct repo URL, `rev` and recommended args