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/modify-config.md

@@ -0,0 +1,75 @@
+---
+source_keys:
+  - context7-pre-commit-com
+  - pre-commit-com
+---
+
+# Modifying an existing `.pre-commit-config.yaml`
+
+Reached from `SKILL.md`'s Route table when the repo already has a config. Self-contained — the
+create flow's file is not needed here. `SKILL.md`'s three common gates still apply.
+
+Read the existing `.pre-commit-config.yaml` before editing. Note any stale `rev` values (see
+**Rev staleness** below) but do not change them.
+
+## Adding a hook
+
+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
+   ```
+
+2. Read `references/hooks-by-language.md` for the correct repo URL, `rev` and recommended args
+   before writing anything.
+
+3. Check for duplicates. If the same hook ID, or an equivalent tool, is already configured, say so
+   and stop rather than adding a second one.
+
+4. To sanity-check a hook against the repo's real files before committing to it, smoke-test it:
+
+   ```bash
+   pre-commit try-repo <repo-url> <hook-id> --verbose
+   ```
+
+   Use a local path in place of the URL for a hook under development. This runs the hook without
+   writing anything.
+
+5. If the hook's source repo is already a block in the config, add the hook under that block.
+   Otherwise append a new repo block.
+
+6. State the proposed addition, wait for confirmation, write, and run `pre-commit validate-config`.
+   On a non-zero exit, show the error, fix it, and re-validate.
+
+## Removing a hook
+
+1. Identify the hook entry and its parent repo block.
+
+2. State what will be removed — the hook ID, and whether the parent repo block goes with it because
+   it would be left with zero hooks. Wait for confirmation.
+
+3. Remove the hook entry. If the repo block now has no hooks left, remove the whole block: an empty
+   `hooks: []` fails `validate-config`.
+
+4. Write, then run `pre-commit validate-config`. On a non-zero exit, **revert the edit**, show the
+   error, and stop. A removal is not safely fixable in place the way a malformed new hook block is,
+   so recovering the prior state beats patching forward.
+
+## Configuring top-level keys
+
+Only when the user explicitly asks. Valid keys: `fail_fast`, `default_stages`,
+`default_language_version`, `minimum_pre_commit_version`, `exclude`, `files`,
+`default_install_hook_types`.
+
+State the proposed change and wait for confirmation before writing.
+
+## Rev staleness
+
+For each repo in the config that also appears in `references/hooks-by-language.md`, compare the
+two `rev` values. Flag a mismatch as potentially outdated and tell the user to run `pc-run` to
+autoupdate. Repos absent from the reference cannot be checked — skip them silently. Do not modify
+any `rev` yourself.
+
+The reference table's own pins go stale between updates, so treat a mismatch as a prompt to check
+rather than proof of staleness. `pre-commit autoupdate`, via `pc-run`, is the authoritative answer
+to what the current `rev` actually is.