feat(bin): add safe identical-squash reconciliation

[homestills]

Intent

Add an explicit operator-invoked helper for reconciling one clean, quiesced Git repository when its checked-out default branch and source-backed origin default branch genuinely diverge but have identical root trees.

Changed files

• bin/fm-reconcile-identical-squash.sh
• tests/fm-reconcile-identical-squash.test.sh
• docs/scripts.md

Safety behavior

The helper refuses dirty, detached, off-default, multi-worktree, active-operation, symbolic-ref, incomplete-history, ambiguous-history, unequal-tree, fetch-failure, and raced states. Source acquisition leaves FETCH_HEAD, configured remote-tracking refs, tags, submodules, and unrelated refs unchanged. A single no-dereference ref transaction preserves the complete old local commit under a deterministic direct ref and moves only the local default branch. Post-commit evidence changes are reported without rolling back through a new claimant. Ordinary fleet synchronization is unchanged.

Compatibility

The fetch and ref-transaction command surface is compatible with Git 2.40, with executable verification on Git 2.40 and a current Git release.

Validation

Executable-interface regression coverage passed, including success, idempotence, refusal paths, fetch invariants, symbolic-ref and expected-old races, bounded source advancement, and post-commit claimant retention. All 10 required CI checks pass, including shell lint, stock macOS Bash compatibility, behavior suites, coverage, and repository invariants.

Risk

High. The helper intentionally moves a local branch, but only after strict predicates are proven. Explicit operator quiescence plus pre- and post-transaction evidence remains the concurrency boundary.

Updates from git push no-mistakes

[homestills]

Hi @kunchenguid — the latest version of this PR is awaiting maintainer approval to run the Require no-mistakes and CI workflows. When you have a chance, could you approve those workflow runs and review the PR? Thank you!