feat(git): add dry_run flag to git_push

[MementoRC]

Summary

Follow-up to #173. git push --dry-run is a general-purpose flag (works with normal pushes, force pushes, and the new --delete path from #173) but git_push has no way to invoke it. Workflows that want to validate a push (especially a destructive --delete) without actually performing it currently have no MCP path.

Proposed change

Add dry_run: bool = False to GitPush and the git_push handler. When True, append --dry-run to the repo.git.push(...) call regardless of which branch is taken (normal / force / delete / refspec).

dry_run: bool = Field(
    False,
    description="Show what would be pushed without actually pushing (git push --dry-run). "
                "Compatible with all push modes including delete and refspec."
)

No new mutual-exclusion rules — --dry-run is compatible with every existing flag.

Why not delete-only

The review feedback that prompted this suggested dry-run specifically for the delete path, but git push --dry-run is general. Scoping it to delete-only would be artificial and would require a second flag later for normal-push dry-runs.

Acceptance

• dry_run=True with normal push → repo.git.push('origin', 'main', '--dry-run')
• dry_run=True with delete=True → repo.git.push('origin', '--delete', 'feature', '--dry-run')
• dry_run=True with refspec='...' → repo.git.push('origin', '...', '--dry-run')
• One test per push mode confirming --dry-run is in the call args
• Return value notes that no remote state was modified

Context

Surfaced during PR #174 review; deferred to keep that PR scoped to the original three issues.