feat: add inkssg serve with live reload

[lucasnevespereira]

Summary

Adds inkssg serve — a local dev server that builds the site, serves it on http://localhost:3000, watches sources, and reloads the browser on every change. Closes the first item under v0.2 in ROADMAP.md.

What's in it

• inkssg.Serve(dir, opts...) library entry point.
• inkssg serve [path] [--addr :port] CLI command.
• Recursive fsnotify watcher with 100ms debounce. Ignores the output dir and dotfiles. Picks up new subdirectories as they're created.
• HTTP file server with an HTML injector that adds a tiny SSE-driven reload script before </body>. Non-HTML responses pass through untouched.
• SSE endpoint at /__inkssg/reload with a 30s keep-alive ping. One broadcast per successful rebuild.
• Graceful shutdown on SIGINT/SIGTERM with a 2s server drain.

Only one new dependency: github.com/fsnotify/fsnotify.

Smoke tested

• go run ./cmd/inkssg serve examples/minimal --addr :3456 → 200 OK, reload script present in served HTML.
• touch examples/minimal/pages/index/content.md → ↻ rebuilt in 2ms in server log, SSE clients receive reload.
• Ctrl-C exits cleanly.

Test plan

• go build ./... passes
• go vet ./... passes
• inkssg serve examples/minimal builds and serves
• Editing pages/<x>/content.md triggers a rebuild and reload
• Editing a local theme file triggers a rebuild and reload
• Editing ink.yaml triggers a rebuild and reload
• Creating a new page directory while the server is running gets picked up
• --addr :4000 overrides the default port
• Ctrl-C exits within ~2s