fix(git-read): reject NUL bytes in paths

[outlier27-cell]

ProblemnnCloses #612. Git path validation accepted NUL bytes, then Node rejected the argv at spawn time with ERR_INVALID_ARG_VALUE instead of OpenPI returning InvalidPathError.nn## ValuennReject unsupported paths at the tool boundary with a stable, actionable error.nn## ApproachnnReject NUL in the shared repository-path validator and cover show, diff, and log argv builders.nn## Validationnn- Red: latest main accepted src\0secret.ts; Node rejected it only at spawn.n- Focused args tests: 4/4 passed.n- Git-read suite: 27/27 passed.n- bun run check: passed.n- bun run test: changed Git-read tests passed in the full run; it then reproduced the known unrelated Windows real-Pi post-edit timeout cluster, so no full-suite pass is claimed.nn## Impactnn- User-visible: NUL paths return existing InvalidPathError.n- Model-visible tools, runtime/lifecycle, persistence: none.n- Compatibility: NUL cannot be passed to Node process argv or name a normal repository path.

[tt-a1i]

Reviewed the NUL path guard and argv-builder coverage at 9b9ad43. No blocking findings; exact-head CI is green.

[tt-a1i]

Rechecked the updated head f88163d against current main. The PR still changes only the NUL path guard and its argv tests; no blocking findings.