refactor(pc-author): retrofit to the ADR-0020 context contract

Description 475 -> 213 chars, body 680 -> 212 words. Create and modify
become self-contained flow files behind a dispatch table, since the two are
mutually exclusive on whether the config already exists.

Passed its clean-context audit with no must-fix findings.

plugins/git/skills/pc-author/references/create-config.md

@@ -0,0 +1,31 @@
+---
+source_keys:
+  - context7-pre-commit-com
+  - pre-commit-com
+---
+
+# Creating a `.pre-commit-config.yaml`
+
+Reached from `SKILL.md`'s Route table when the repo has no config yet. Self-contained — the modify
+flow's file is not needed here. `SKILL.md`'s three common gates still apply.
+
+## Steps
+
+1. Detect what languages are actually in the repo with a shallow extension scan, rather than
+   inferring them from the project's name or README:
+
+   ```bash
+   git ls-files | grep -oE '\.[a-z]+$' | sort | uniq -c | sort -rn
+   ```
+
+2. Read `references/hooks-by-language.md` and map the detected extensions to recommended hooks.
+   Take the repo URL, `rev` and args from that file rather than from memory — a `rev` that does not
+   exist is the most common way a fresh config fails on its first run.
+
+   For a deliberately minimal starting point instead of a full recommendation set,
+   `pre-commit sample-config > .pre-commit-config.yaml` prints a small starter config to build on.
+
+3. State the proposed config in full and wait for the user's confirmation.
+
+4. Write `.pre-commit-config.yaml`, then run `pre-commit validate-config`. If it exits non-zero,
+   show the error, fix it in place, and re-validate.