feat: support Brave + Chrome channels, fix stale DevToolsActivePort discovery (#13)

* feat: support Brave + Chrome channels, fix stale DevToolsActivePort discovery

Broadens browser detection and hardens CDP endpoint discovery, based on the
investigation and approach in #4 (Brave Beta on macOS with a leftover Chrome
DevToolsActivePort file).

Browser detection (src/setup.ts, src/cdp/discovery.ts):
- checkChromeRunning() now recognizes Brave (macOS "brave browser", Linux
  brave/brave-browser, Windows brave.exe), alongside the existing Chrome/
  Chromium/Edge detection. Also excludes gpu/updater sub-processes that can
  linger after a browser quits.
- profileDirs() gains Brave (Stable/Beta/Nightly/Dev) and Chrome Beta/Dev/
  Canary/Chromium profile paths across macOS, Linux, and Windows.

Stale DevToolsActivePort handling (src/cdp/discovery.ts):
Previously discovery trusted the first readable DevToolsActivePort file. A
browser that has quit leaves its file behind, so if another browser later binds
the same port (e.g. 9222), discovery returned a WS URL carrying the dead
browser's UUID against the live browser's server, and the connection failed.
discoverWsUrl() now:
- collects all readable candidates instead of committing to the first;
- when candidates share a port, keeps the most-recently-written file (mtime);
- skips candidates whose port isn't live via a fast single-shot probe (no
  30s waitForPort block on stale files);
- asks each live browser for its canonical webSocketDebuggerUrl via
  /json/version, falling back to the file's WS path when that endpoint is
  disabled (some browsers only expose it via --remote-debugging-port);
- falls back to probing well-known ports (9222, plus BU_CDP_PORTS) when no
  profile file is readable — covering sandboxed harnesses (EPERM/EACCES) and
  non-default install locations.

Setup guidance updated to mention brave://inspect / edge://inspect and the
--remote-debugging-port launch flag.

Verified: typecheck clean; queryLiveWsUrl / mtime-dedup / fallback-port logic
exercised against a mock /json/version server; checkChromeRunning() matches the
single main process against live macOS ps output, excluding all helpers.

Closes #4

Co-authored-by: Trevor Whitney <trevorjwhitney@gmail.com>

* fix: harden discovery against malformed port files + stale-then-fallback

Addresses two issues raised in review of the discovery rewrite:

- Validate the port parsed from DevToolsActivePort before building a
  candidate. A truncated/corrupt file (non-numeric or out-of-range first
  line) yielded NaN, and net.connect throws ERR_SOCKET_BAD_PORT
  synchronously for NaN / <=0 / >=65536 — which escaped discoverWsUrl as an
  unhandled rejection instead of surfacing as a Result error. Such entries
  are now skipped.

- Fall back to well-known ports (9222 / BU_CDP_PORTS) after all discovered
  candidates prove stale. Previously the fallback ran only when no port file
  was readable, so a leftover file for a quit browser caused a ~30s
  waitForPort block and failure without ever trying a live fallback port.
  Ports already covered by a candidate are skipped to avoid re-probing.

---------

Co-authored-by: Trevor Whitney <trevorjwhitney@gmail.com>

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