Forward host timezone to guest shell

[semistrict]

Summary

The lnx guest shell defaulted to UTC even when the host was in another timezone. $TZ was already forwarded to guest execs when set, but macOS hosts rarely set it — the timezone lives in the /etc/localtime symlink, not the environment.

When $TZ is unset, forwarded_exec_env now falls back to detecting the host zone: it resolves /etc/localtime (e.g. …/zoneinfo/America/New_York), with /etc/timezone as a Debian fallback, and injects it as TZ for every guest exec including the login shell. Deterministic mode still pins TZ=UTC.

Test plan

• Unit test zone_from_localtime_target_parses_zoneinfo_paths covers the symlink-target parsing (passes via bun run test).
• End-to-end on a fresh instance: lnx run -- sh -c 'echo TZ=$TZ; date' now prints TZ=America/New_York / EDT, where before the guest reported UTC.

Notes

• The fix is computed in the long-lived host broker process, so an already-running instance's broker must restart before it picks up the change.