test(repo-mode): add regression coverage for issue #85

[jhapriyavart10]

Summary

Adds hermetic regression coverage for the four repo-mode safety properties requested in Issue #85:

• explicit repo directory required (initLocalSession validation and query() system error event)
• ancestor repository escape prevention (refuses execution in child directories without local .git)
• mismatched origin protection (refuses to overwrite existing repositories tracking a different remote)
• readOnly mutation prevention with credential scrubbing (skips auto-commits and branch push on finalize, while stripping authentication tokens from origin)

Validation

• npm run build passes
• existing 75 tests pass
• new suite: 6 tests
• current main: 5 expected failures because the safety implementation in PR fix(repo-mode): prevent unsafe dir defaults and forced pushes (#85) #88 is not yet merged
• PR fix(repo-mode): prevent unsafe dir defaults and forced pushes (#85) #88 worktree: 6/6 regression tests pass

Context

This is intentionally a companion test PR for the existing maintainer implementation in #88. No production code is changed.

Testing

All repository operations in the new tests use isolated temporary local Git repositories/bare remotes; no external network or real credentials are required.

Related to #85
Companion to #88