Refactor browser tool PATH construction to include Termux directories
(/data/data/com.termux/files/usr/bin, /data/data/com.termux/files/usr/sbin)
so agent-browser and npx are discoverable on Android/Termux.
Extracts _browser_candidate_path_dirs() and _merge_browser_path() helpers
to centralize PATH construction shared between _find_agent_browser() and
_run_browser_command(), replacing duplicated inline logic.
Also fixes os.pathsep usage (was hardcoded ':') for cross-platform correctness.
Cherry-picked from PR #9846.
On macOS with Homebrew (Apple Silicon), Node.js and agent-browser
binaries live under /opt/homebrew/bin/ which is not included in the
_SANE_PATH fallback used by browser_tool.py and environments/local.py.
When Hermes runs with a filtered PATH (e.g. as a systemd service),
these binaries are invisible, causing 'env: node: No such file or
directory' errors when using browser tools.
Changes:
- Add /opt/homebrew/bin and /opt/homebrew/sbin to _SANE_PATH in both
browser_tool.py and environments/local.py
- Add _discover_homebrew_node_dirs() to find versioned Node installs
(e.g. brew install node@24) that aren't linked into /opt/homebrew/bin
- Extend _find_agent_browser() to search Homebrew and Hermes-managed
dirs when agent-browser isn't on the current PATH
- Include discovered Homebrew node dirs in subprocess PATH when
launching agent-browser
- Add 11 new tests covering all Homebrew path discovery logic
