feat: stop a single browser by name

[johan44co]

Closes #107.

Summary

Adds an optional NAME positional argument to dev-browser stop:

dev-browser stop          # daemon-wide stop (unchanged)
dev-browser stop <name>   # stop just that browser, leave the daemon up

The daemon already implements browser-stop (protocol.ts, daemon.ts, browserManager.stopBrowser); this just exposes it on the CLI. Stopping an unknown name is a silent no-op, matching the daemon's idempotent behavior.

Why

Without this, any workflow that uses unique --browser names (e.g. timestamped names for test isolation) leaks an instance per run — there's no way to clean them up except dev-browser stop, which also kills every other browser. See #107 for a full reproducer.

Diff

Single file, ~25 lines of Rust:

• Command::Stop → Command::Stop { name: Option<String> }
• Match arm branches on name:
  • None → existing {type: "stop"} flow (daemon kill)
  • Some(name) → new {type: "browser-stop", browser: name} flow
• Help text updated to describe both modes.

No daemon changes. No new dependencies.

Verification

$ dev-browser browsers
NAME             TYPE      STATUS   PAGES
smoke-a          launched  running  login
smoke-b          launched  running  login
tabs-experiment  launched  running  page-a, page-b

$ dev-browser stop tabs-experiment
Stopped browser 'tabs-experiment'.

$ dev-browser browsers
NAME      TYPE      STATUS   PAGES
smoke-a   launched  running  login
smoke-b   launched  running  login

$ dev-browser stop unknown-name
Stopped browser 'unknown-name'.   # idempotent no-op, exit 0

$ dev-browser stop --help
Stop the daemon, or a single managed browser

Usage: dev-browser stop [NAME]

Arguments:
  [NAME]
          Optional name of a managed browser to stop. Without this, stops
          the daemon and all browsers.

Local CI checks pass: cargo fmt -- --check, cargo check, cargo build.

I left CHANGELOG.md alone since the maintainer manages versioning/release entries — happy to add an entry under an Unreleased heading if preferred.