feat: mds watch command (file watcher with auto-recompile)

[dean0x]

Summary

Add an mds watch command that monitors .mds files for changes and automatically recompiles on save. Essential for iterative prompt development workflows.

Motivation

Prompt engineering is an iterative process — edit template, compile, review output, repeat. Currently users must manually run mds build after every edit. A watch mode removes this friction and enables a live-preview workflow when combined with a Markdown viewer.

Proposed CLI

mds watch template.mds                          # Watch single file + its imports
mds watch template.mds -o output.md             # Auto-recompile to specific output
mds watch .                                     # Watch all .mds files in directory
mds watch template.mds --vars vars.json         # With runtime variables
mds watch template.mds --set name=Alice         # With --set overrides
mds watch template.mds --clear                  # Clear terminal before each rebuild

Design Considerations

• Dependency tracking: When a.mds imports b.mds, changes to b.mds should trigger recompilation of a.mds. The CompileOutput.dependencies field already provides the import graph.
• Debouncing: File writes often produce multiple filesystem events — debounce with a ~100ms window
• Error display: On compilation error, print the diagnostic to stderr but keep watching (don't exit). On next successful compile, clear the error.
• Performance: For single-file watch, use OS-native file watching (inotify/kqueue/ReadDirectoryChanges). For directory watch, use recursive watching with .mds extension filter.
• Signal handling: Ctrl+C exits cleanly
• Rust crate: notify is the standard cross-platform file watcher
• Output format: Reuse all mds build output resolution logic (stdout, -o, --out-dir)
• mds.json integration: watch respects build.output_dir from project config

Acceptance Criteria

• mds watch <file> recompiles on save
• Import dependencies are tracked and trigger recompilation
• Errors are displayed without exiting watch mode
• All mds build output options work with mds watch
• Ctrl+C exits cleanly
• --clear flag clears terminal between rebuilds
• Debouncing prevents duplicate compilations