fix(install): add OG_POLICY_PATH fallback for the policies .pth

[jmvbambico]

Problem

install_pth() writes a .pth file into omnigent's site-packages so
omnigent_local_policies is importable by the host daemon. That path lives
inside the venv's versioned prefix (e.g. Homebrew's Cellar/omnigent/<ver>/),
which a package-manager upgrade replaces wholesale rather than patching in
place — orphaning the .pth. The result: every policy evaluation raises
ModuleNotFoundError, and the failure mode is deny-everything, with nothing
in the chat error pointing back to a stale .pth.

Fix

og.env lives outside the versioned venv prefix and survives upgrades, so:

• write_og_env() now writes OG_POLICY_PATH=<policies dir> into og.env.
• og defaults OG_POLICY_PATH to $OMNI_HOME/policies (so an og.env
  written before this change still gets the fallback), and exports it as
  PYTHONPATH (prepended, not replacing any existing PYTHONPATH) when
  start_host_daemon() launches omnigent host.

This makes the .pth redundant rather than load-bearing: it's still written
and verified as before, but if a future upgrade orphans it, the host daemon
still finds the policy module via PYTHONPATH.

Testing

Reproduced the failure locally (Homebrew brew upgrade omnigent replaced the
Cellar prefix, orphaning the .pth, "every message denied"). Verified the fix
by re-applying the saved install plan (./install.sh --plan ~/.omnigent/og-install.json)
and confirming OG_POLICY_PATH lands in og.env and the PYTHONPATH export
lands in the installed og script.

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