fix(specgit): restore delivery state after failed bootstrap

[LeXwDeX]

This was generated by AI during triage.

Why

A failed SpecGit bootstrap during #519 left .specgit.yaml deleted in the worktree before the next validation failure. Bootstrap is not transactional: a nonzero exit can leak partial local delivery state and make the following attempt diagnose the residue instead of the original error.

Scope

• Make the repository-local bootstrap path snapshot and restore pre-existing delivery state when SpecGit exits nonzero.
• Cover .specgit.yaml and every generated file temporarily swapped by chore(specgit): automate bootstrap around required harness specializations #521.
• Preserve the original exit code and diagnostic.

Out of scope: hiding a successful bootstrap's intended binding changes or deleting user-authored worktree changes.

Acceptance

• Forced failures at validation, branch creation, push, and PR creation leave all pre-existing tracked and untracked delivery files byte-identical.
• Successful bootstrap keeps the new .specgit.yaml binding and restores only temporary harness substitutions.
• The wrapper refuses to start when it cannot distinguish its temporary files from pre-existing user changes.

Dependency

• Blocked by chore(specgit): automate bootstrap around required harness specializations #521's safe repository-local bootstrap seam.

[LeXwDeX]

This was generated by AI during triage.

Agent brief: the issue body is the verified delivery contract. Implement only after #521 provides the safe repository-local bootstrap seam; preserve all branch rulesets and SpecGit acceptance gates.

[LeXwDeX]

SpecGit delivery branch: feat/530-specgit-bootstrap-rollback (draft pull request #533).

[github-actions]

Auto-closed: delivery PR #533 (view) merged into dev with a closing keyword for #530 in its body. GitHub's native auto-close only fires on the default branch (main); this mirrors it for the dev integration layer (#519).