fix: add macOS Homebrew paths to browser and terminal PATH resolution

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

tools/environments/local.py

@@ -254,7 +254,12 @@ def _clean_shell_noise(output: str) -> str:
     return result
 
 
-_SANE_PATH = "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+# Standard PATH entries for environments with minimal PATH (e.g. systemd services).
+# Includes macOS Homebrew paths (/opt/homebrew/* for Apple Silicon).
+_SANE_PATH = (
+    "/opt/homebrew/bin:/opt/homebrew/sbin:"
+    "/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
+)
 
 
 def _make_run_env(env: dict) -> dict: