guard: a task never writes outside its ground, and every refusal is true for the path it names #78

Path-keyed guard: write/edit/bash cd/git aimed outside the task's ground are refused as "outside your copy — your ground is X", never with the worktree sentences (taskgit.go said "reaches no remote… comes home on its own" about the person's real checkout). git push to any remote and gh api mutations from a task are refused the same way (the bypass in #76). Ground = the worktree dir until #76's engine lane lands; read the field when it exists.

Part of #76. Branch ground/guard → PR into feat/task-ground.

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