feat: validation gates stop hook (#7)

[elynch303]

Closes #7

Changes

New: .claude/hooks/stop/validation_gates.py

• Auto-detects stack from package.json, pyproject.toml, go.mod, Cargo.toml
• Runs enabled gates in sequence: lint → security → typecheck → build (+ optional tests/e2e/ui)
• Checks tool availability with which before running each gate — skips silently if not installed
• Only activates when git status shows modified files — no noise on read-only sessions
• Returns exit 2 with structured stderr on failures so Claude sees them and can invoke validation-gates agent to fix
• Returns exit 0 if clean, no tools, no stack, or unexpected error — never crashes a session
• stdlib only, under 500 lines

New: tests/hooks/test_validation_gates.py

• 26 unit tests (stdlib unittest)
• Covers: stack detection, gate enable/disable from config, clean exit when no tools, output format, exit codes

Modified: .claude/settings.json.example

• Added stopGates config block with all keys and defaults
• Hook registered in the Stop hooks array

Modified: README.md + scripts/install.sh

• Validation Gates entry added to Safety Hooks table
• Install note pointing users to stopGates config

Test plan

• Session with modified files + ruff installed → lint failures surface to Claude
• Session with no modified files → hook exits 0 silently
• stopGates.lint: false in settings.json → lint gate skipped
• No linter installed → gate skipped, exit 0
• python -m unittest discover -s tests/ → 26 tests pass

🤖 Generated with Claude Dev Kit