Fix Unicode goal launches without an inherited locale

[scgopi]

When graphcoded inherits no locale from launchd, Unicode in a goal can be consumed as shell editing keystrokes before the backend starts. Default an absent or empty LANG to UTF-8 in the PTY environment, before zmx starts its receiving shell. Existing locale settings and per-call environment overrides retain precedence.

Closes #344.

Validation	Result
Full macOS app suite	1,753 tests in 185 suites passed; xcodebuild exit 0 and TEST SUCCEEDED
SwiftPM CLI and daemon build	Passed
Unicode end-to-end probe	5/5 locale cases passed for goal argv and node send
Unmodified main control (18929a9)	Same probe fails with no locale: recorder never starts
SwiftLint	0 errors; 193 warnings
Swift formatting and git diff --check	Passed

To reproduce on macOS from this branch:

swift build --scratch-path .spm
python3 scripts/unicode-launch-probe.py \
  .spm/debug/graphcode .spm/debug/graphcoded \
  "$HOME/.graphcode/bin/zmx"

Use the path to your installed zmx if it lives in a different support workspace. The probe runs the real CLI, isolated daemon, and zmx with temporary HOME, shell configuration, socket directory, and graph state. A fake Claude executable records argv and terminal input without making model requests. It checks en/em dashes, an arrow, emoji, a combining accent, and Japanese text under missing/empty LANG and explicit UTF-8 locale settings; all five cases should print PASS. Evidence directories are printed and retained; the daemon and its session are stopped.

Coverage is new local PTY sessions. Existing sessions, SSH hosts, and real Claude TUI/model behavior were not exercised. Explicit non-UTF-8 locale settings are preserved. Linux uses C.UTF-8 but was not tested here.