Add `/lint` skill with `scripts/check.py`

Add a user-invocable `/lint` skill that wraps
`ruff check` with staged-file defaults and piker's
`ruff.toml` config via a PEP 723 script.

The `ruff.toml` (from prior commit) now enforces
piker's `'''` multiline docstring convention with
`D2xx` formatting rules: summary on second line
(D213), closing quotes on separate line (D209), blank
line between summary and description (D205), plus
`W` trailing-whitespace and cherry-picked `D4xx`
content checks.

Deats,
- `scripts/check.py`: pure-stdlib wrapper that
  defaults to staged `.py` files via
  `git diff --cached`, with `--all`, `--fix`,
  `--diff`, and `--stats` flags.
- calls `ruff` from `$PATH` (works with nixpkgs,
  nvim-bundled, or pip-installed).
- prints clear install guidance if `ruff` not found.
- `SKILL.md`: documents usage, common violations
  (D213, D205, D204), and fix patterns for
  converting to piker's docstring style.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

.claude/skills/lint/SKILL.md

@@ -0,0 +1,72 @@
+---
+name: lint
+description: >
+  Run ruff lint checks on piker Python files for
+  docstring style (D2xx) and code convention
+  compliance. Checks staged files by default.
+user-invocable: true
+argument-hint: "[--all|--fix|--stats|paths...]"
+allowed-tools: Bash(python3 *), Bash(ruff *), Read, Edit
+---
+
+# Ruff Lint Checker
+
+Run piker's ruff config against staged files,
+specific paths, or the full codebase.
+
+## Available scripts
+
+- **`scripts/check.py`** — self-contained (PEP 723)
+  wrapper around `ruff check` that defaults to staged
+  files and uses the project's `ruff.toml`.
+
+## Usage
+
+```bash
+# check staged Python files (default)
+python3 scripts/check.py
+
+# check full codebase
+python3 scripts/check.py --all
+
+# auto-fix fixable violations
+python3 scripts/check.py --fix
+
+# preview fixes without applying
+python3 scripts/check.py --diff
+
+# show per-rule violation counts
+python3 scripts/check.py --stats
+
+# check specific files
+python3 scripts/check.py piker/ui/_style.py
+```
+
+## Common violations
+
+| Rule | Meaning | Fixable? |
+|------|---------|----------|
+| D213 | summary not on 2nd line after `'''` | yes |
+| D205 | no blank line after summary | no |
+| D204 | no blank line after class docstring | yes |
+| D209 | closing quotes not on own line | yes |
+| D200 | ignored — piker always multiline | n/a |
+| W291 | trailing whitespace | yes |
+
+## Fixing D213 (most common)
+
+Convert this:
+```python
+"""Summary on first line."""
+```
+
+To piker's `'''` multiline style:
+```python
+'''
+Summary on second line.
+
+'''
+```
+
+For D205, insert a blank line between the summary
+and description paragraphs inside the docstring.

.claude/skills/lint/scripts/check.py

@@ -0,0 +1,130 @@
+#!/usr/bin/env python3
+# /// script
+# requires-python = ">=3.12"
+# dependencies = []
+# ///
+'''
+Ruff lint checker for piker docstring and code style.
+
+Checks staged files by default, or the full `piker/`
+tree with `--all`. Uses the project's `ruff.toml`.
+
+'''
+import argparse
+import shutil
+import subprocess
+import sys
+
+
+def get_staged_py_files() -> list[str]:
+    '''
+    Return staged Python file paths (added/copied/modified).
+
+    '''
+    result = subprocess.run(
+        [
+            'git', 'diff',
+            '--cached',
+            '--name-only',
+            '--diff-filter=ACM',
+            '--', '*.py',
+        ],
+        capture_output=True,
+        text=True,
+    )
+    return [
+        f for f in result.stdout.strip().split('\n')
+        if f
+    ]
+
+
+def main() -> int:
+    '''
+    Parse args and run `ruff check` against targets.
+
+    '''
+    ap = argparse.ArgumentParser(
+        description=(
+            "Run ruff check with piker's ruff.toml config.\n"
+            '\n'
+            'Default: checks staged .py files only.\n'
+            'Use --all for the full piker/ tree.'
+        ),
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    ap.add_argument(
+        'paths',
+        nargs='*',
+        help='files/dirs to check (default: staged)',
+    )
+    ap.add_argument(
+        '--all', '-a',
+        action='store_true',
+        dest='check_all',
+        help='check entire piker/ directory',
+    )
+    ap.add_argument(
+        '--fix',
+        action='store_true',
+        help='auto-fix fixable violations',
+    )
+    ap.add_argument(
+        '--diff',
+        action='store_true',
+        help='preview fixes as unified diff (dry-run)',
+    )
+    ap.add_argument(
+        '--stats',
+        action='store_true',
+        help='show per-rule violation counts only',
+    )
+    args = ap.parse_args()
+
+    # determine check targets
+    if args.check_all:
+        targets: list[str] = ['piker/']
+    elif args.paths:
+        targets = args.paths
+    else:
+        targets = get_staged_py_files()
+        if not targets:
+            print(
+                'No staged Python files found.\n'
+                'Use --all for full codebase '
+                'or pass file paths.'
+            )
+            return 0
+        print(
+            f'Checking {len(targets)} staged file(s)..\n'
+        )
+
+    # ensure ruff is available on PATH
+    if not shutil.which('ruff'):
+        print(
+            'Error: `ruff` not found on PATH.\n'
+            'On NixOS: add to flake.nix or '
+            '`nix-shell -p ruff`\n'
+            'Otherwise: `uv sync --group lint` or '
+            '`pip install ruff`'
+        )
+        return 127
+
+    # build ruff command
+    cmd: list[str] = ['ruff', 'check']
+
+    if args.diff:
+        cmd.append('--diff')
+    elif args.fix:
+        cmd.append('--fix')
+
+    if args.stats:
+        cmd.append('--statistics')
+
+    cmd.extend(targets)
+
+    result = subprocess.run(cmd)
+    return result.returncode
+
+
+if __name__ == '__main__':
+    sys.exit(main())