fix(skill): add pre-commit and duplicate-PR checks to running-in-ci

The claude-mention bot created a duplicate PR (#1442) when #1416 already
addressed the same issue, and shipped code without running cargo fmt —
causing a CI failure and requiring a second commit. Both gaps exist because
running-in-ci (loaded by all CI workflows) had no guidance on checking for
existing PRs or formatting code before committing. The triage-issue skill
had these checks but running-in-ci did not.

Add two subsections under "PR Creation":
- "Before creating a PR" — check open PRs and fix branches first
- "Before committing code changes" — run cargo fmt at minimum

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: worktrunk-bot

.claude/skills/running-in-ci/SKILL.md

@@ -40,6 +40,35 @@ When the triggering comment asks for a PR (e.g., "make a new PR", "open a PR",
 "create a PR"), create it directly with `gh pr create`. The comment is the
 user's explicit request — don't downgrade it to a compare link.
 
+### Before creating a PR
+
+**Check for existing work.** Before creating a new branch or PR, check whether
+someone (including the bot) has already opened a PR for the same issue or topic:
+
+```bash
+# Check open PRs — look for related titles and branches
+gh pr list --state open --json number,title,headRefName --jq '.[] | "#\(.number) [\(.headRefName)]: \(.title)"'
+
+# Check for fix branches
+git branch -r --list 'origin/fix/*'
+```
+
+If an existing PR addresses the same problem, work on that PR instead of
+creating a duplicate. Comment on the existing PR or the issue linking to it.
+
+### Before committing code changes
+
+Always run `cargo fmt` before committing Rust code changes. Ideally run the full
+lint and test suite:
+
+```bash
+cargo fmt
+cargo run -- hook pre-merge --yes   # full tests + lints
+```
+
+At minimum, `cargo fmt` prevents formatting-only CI failures that waste a round
+trip (push, wait for CI, fix formatting, push again, wait again).
+
 ## Fork PRs
 
 Before pushing commits to a PR branch, check whether it's a fork PR: