test: mock Claude CLI integration tests + dead export cleanup

[JonyanDunh]

Summary

Closes the coverage gap around the real `spawnSync('claude', ...)` subprocess code path in `hooks/stop-hook.js`, and cleans up 4 dead exports flagged by a grep audit.

Why

Before this PR, the 53 tests in `test/stop-hook.test.js` exercised every branch of the stop hook except the one where it actually spawns `claude -p --model haiku ...`. Every test deliberately took the "no tool uses", "max iterations", "missing transcript", "recursion guard", or "corrupt state" branches to avoid burning real Haiku tokens in CI. That left the single riskiest path — the Windows `cmd.exe` shell-quoted subprocess invocation with complex JSON prompts — completely untested.

What

New test file: `test/stop-hook-haiku.test.js` (6 tests)

Creates a temporary bin directory with a cross-platform mock Claude CLI:

• `fake-claude.js` — shared Node.js implementation. Distinguishes `claude --version` (used by the `claudeCliAvailable()` pre-flight check, always succeeds) from `claude -p` (the real verdict call, emits a verdict controlled by the `WATCHDOG_FAKE_HAIKU_VERDICT` env var).
• `claude` — POSIX shell wrapper (`#!/bin/sh` + exec node), resolved by `spawnSync('claude', ...)` on Linux/macOS.
• `claude.cmd` — Windows batch wrapper, resolved via `PATHEXT` when the hook uses `shell: true` on Windows.

The test prepends this bin dir to `PATH` and runs the real `hooks/stop-hook.js` subprocess with realistic `HOOK_INPUT` + state file + transcript, then asserts on every verdict branch:

Verdict	Expected behavior
`FILE_CHANGES`	`{decision: "block"}`, iteration bumped, state preserved
`NO_FILE_CHANGES`	State file removed, allow stop
Ambiguous (both markers)	Continue loop as safety
Ambiguous (neither)	Continue loop as safety
CLI failure (exit 1)	Continue loop as safety

Plus a sanity test that the mock is actually resolvable on PATH.

Dead export cleanup

`lib/constants.js`: `STATE_DIR`, `STATE_FILE_PREFIX`, `STATE_FILE_SUFFIX` were used internally by `stateFilePath()` but exported to no one. Demoted to module-local `const`.

`lib/judge.js`: `claudeCliAvailable()` is used internally by `askHaiku()` but exported to no one. Removed from `module.exports`.

Doc updates

• README test count: 53 → 59 across all 7 languages
• CI workflow explicit file list includes the new test file
• CONTRIBUTING.md describes the new test file and mock-CLI approach

Test plan

• `node --test ...` locally — 59/59 passing (up from 53)
• New test file found the real bug in my own fake script (fake`--version` was being affected by the FAIL env var, routing hook into CLI_MISSING instead of CLI_FAILED). Fixed.
• CI will confirm across 3 OSes × 3 Node versions = 9 matrix jobs

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com