refactor: rename commit skill to commit-staged

The folder name now matches the  field in the SKILL.md frontmatter.
Updated links in the root, common/, and engineering/ READMEs.

common/engineering/commit-staged/SKILL.md

@@ -0,0 +1,30 @@
+---
+name: commit-staged
+description: Commit staged files with a conventional commit message.
+disable-model-invocation: true
+---
+
+Commit staged files with a conventional commit message.
+
+## Process
+
+1. **Check the staging area** — Run `git diff --cached --stat`. If empty, report "nothing staged" and stop.
+
+   Completion criterion: Staged files are listed, or the skill terminates.
+
+2. **Draft the message** — Inspect `git diff --cached` to understand what changed. Write a [conventional commit](https://www.conventionalcommits.org/) message:
+
+   - Format: `type(scope): summary` — scope is optional.
+   - Common types: `feat`, `fix`, `refactor`, `docs`, `chore`, `test`, `style`, `perf`, `ci`, `build`.
+   - Summary: imperative mood, lowercase, no period, ≤72 characters.
+   - Body (if needed): wrap at 72 characters, explain _what_ and _why_, not _how_.
+
+   Completion criterion: A conventional commit message is composed.
+
+3. **Commit** — Run `git commit -m "<message>"`. If the commit fails, report the error and stop.
+
+   Completion criterion: `git commit` exits 0.
+
+4. **Report** — Print the short commit hash and the first line of the message.
+
+   Completion criterion: The hash and message are printed.