Harden git ref handling against argument injection

[Copilot]

A network-reachable git argument-injection path remained in ref-based workflows: user-controlled refs were passed directly to git, so values beginning with - could be parsed as flags rather than revisions.

• Security hardening

  • Added a shared git-ref validation helper that rejects unsafe refs before invoking git.
  • Applied the guard to the ref-resolution paths used by semantic bisect, temporal time filtering, branch-diff resolution, and narrator-side git lookups.

• Regression coverage

  • Added focused tests covering unsafe refs and asserting they are rejected before git is invoked.

if (!isSafeGitRange(ref)) {
  throw new Error('Unsafe git ref')
}

[jsilvanus]

@copilot is the review fixes done all?

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