chore(specgit): automate bootstrap around required harness specializations

[LeXwDeX]

Closes #521

Why

The pinned SpecGit 1.10.1 currency gate rejects every specgit issue run because this repository intentionally specializes the generated acceptance workflow. Refreshing the harness manually is unsafe because it overwrites those required bytes.

What changed

• Added script/specgit-bootstrap.sh as the canonical repository-local entry point.
• Snapshot and restore the complete SpecGit init write surface on success, failure, and handled signals.
• Fail closed for dirty write-surface paths, missing binding state, and restore hash mismatches.
• Preserve wrapped arguments, diagnostics, exit status, and the single-JSON stdout contract.
• Added isolated fake-CLI behavior tests and updated repository instructions.

Evidence

• bash script/specgit-bootstrap.test.sh: 38 passed, 0 failed.
• sh -n script/specgit-bootstrap.sh: passed.
• bash -n script/specgit-bootstrap.test.sh: passed.
• shellcheck -s sh script/specgit-bootstrap.sh: passed.
• shellcheck -s bash script/specgit-bootstrap.test.sh: passed.
• Commit hooks: root lint passed at 4840/4850 warnings; all 29 typecheck tasks passed.
• DAG verification: PASS; corrected diff review: PASS with 0 blocking findings.

Checklist

• Why, What changed, and Evidence are filled in.
• specgit finish exits 0.