feat: add watch mode subcommand

[SauronBot]

Summary

Adds bashunit watch [path] [options] — a new subcommand that watches .sh files for changes and automatically re-runs tests on save, enabling a fast TDD feedback loop without manual re-runs.

This was a frequently missing feature compared to test runners in other ecosystems (Jest --watch, PHPUnit with file watchers, etc.).

How it works

1. Runs tests immediately on start
2. Watches the given path for .sh file changes
3. On change detected, re-runs the full test suite
4. Loops until interrupted with Ctrl-C

Implementation

• src/watch.sh — core watch loop, supports both inotifywait (Linux) and fswatch (macOS)
• src/main.sh — cmd_watch() subcommand handler, forwards extra args to bashunit test
• bashunit — sources watch.sh, registers watch in subcommand detection and router
• src/console_header.sh — help text for watch --help, listed in main --help

Usage

bashunit watch                       # watch current directory
bashunit watch tests/                # watch specific path
bashunit watch tests/ --filter user  # watch and filter by test name
bashunit watch tests/ --simple       # minimal output

Requirements

Platform	Tool	Install
Linux	inotifywait	sudo apt install inotify-tools
macOS	fswatch	brew install fswatch

When neither tool is present, a clear error message with install instructions is shown.

Testing

All 603 existing unit tests pass with no changes (bashunit tests/unit/ --simple).

Manual testing:

• Verified bashunit watch --help output
• Verified bashunit --help lists the new command
• Verified immediate test run on start
• Verified re-run triggers on .sh file save (Linux with inotifywait)
• Verified clean Ctrl-C exit