Account for env -C and env --chdir Before Resolving GIT_DIR in the Write Guard #3716

Raised by the review of promotion #3715 on the change #3709 made, as a P1 thread.

_launch in host-setup/agent-safety/claude/gh-write-guard.py strips env -C <dir> and env --chdir=<dir> from a command without changing the directory the command runs in. Requirement 4 then resolves a relative GIT_DIR, and the remote, from the caller's repository. Constructed shape:

env --chdir=/srv/other GIT_DIR=.git git push origin main

git runs in /srv/other and pushes that repository's origin, while the guard checks the working directory's. The fix is to carry the launcher's directory into the command model, or to read the shape as an undetermined repository, which blocks main, master and develop.

Backlog burndown claim. Branch: feature/burndown-guard-env-chdir. Predicted file set: host-setup/agent-safety/claude/gh-write-guard.py, host-setup/agent-safety/README.md.