feat(claude): add shfmt/vendor hooks and shell-validate skill

.claude/skills/shell-validate/SKILL.md

@@ -0,0 +1,37 @@
+---
+name: shell-validate
+description: Validate shell scripts after editing. Apply when writing or modifying any shell script in local/bin/ or scripts/.
+user-invocable: false
+allowed-tools: Bash, Read, Grep
+---
+
+After editing any shell script in `local/bin/`, `scripts/`, or `config/` (files with a `#!` shebang or `# shellcheck shell=` directive), validate it:
+
+## 1. Determine the shell
+
+- `/bin/sh` or `#!/usr/bin/env sh` shebang -> POSIX, use `sh -n`
+- `/bin/bash` or `#!/usr/bin/env bash` shebang -> Bash, use `bash -n`
+- `# shellcheck shell=bash` directive (no shebang) -> use `bash -n`
+- `# shellcheck shell=sh` directive (no shebang) -> use `sh -n`
+- No shebang and no directive -> default to `bash -n`
+
+## 2. Syntax check
+
+Run the appropriate syntax checker:
+
+```bash
+bash -n <file>   # for bash scripts
+sh -n <file>     # for POSIX sh scripts
+```
+
+If syntax check fails, fix the issue before proceeding.
+
+## 3. ShellCheck
+
+Run `shellcheck <file>`. The project `.shellcheckrc` already disables SC2039, SC2166, SC2154, SC1091, SC2174, SC2016. Only report and fix warnings that are NOT in that exclude list.
+
+## Key files to never validate (not shell scripts)
+
+- `local/bin/fzf-tmux` (vendor file)
+- `*.md` files
+- `*.bats` test files (Bats, not plain shell)