Fleet: include Homebrew tools in spawned-session PATH

[willwashburn]

Problem

Relay-spawned sessions on Apple Silicon macOS lack /opt/homebrew/bin in
PATH.

As a result, common operator tools installed by Homebrew are unavailable even
though they work in the launching user's normal shell. The fleet-wide observed
failure is gh: command not found, which blocks residents that need GitHub CLI
for repository work.

Expected behavior

Spawned sessions should receive a predictable executable search path that
includes the platform's standard Homebrew prefix, or should faithfully preserve
the safe path supplied by the parent session.

Acceptance criteria

• Include /opt/homebrew/bin for spawned sessions on Apple Silicon macOS
  without dropping existing safe path entries.
• Keep path construction deterministic across local node and Fleet spawn
  routes.
• Do not add arbitrary repository- or world-writable directories.
• Add a regression that spawns a session and resolves an executable installed
  under /opt/homebrew/bin.
• Surface the effective path source in verbose diagnostics without printing
  unrelated environment secrets.