fix(specgit): target delivery pull requests at dev

[LeXwDeX]

Closes #528

Why

SpecGit creates delivery pull requests against the repository default branch, while this fork requires all delivery branches to enter through dev.

What changed

• after a successful specgit issue, discover the unique bound PR from .specgit.yaml
• verify baseRefName, correct a wrong base with gh pr edit --base dev, and re-verify before exit 0
• fail closed on missing, ambiguous, malformed, unreadable, unavailable, edit-failed, or unverifiable PR state while retaining the successful binding for retry
• add hermetic gh stubs and coverage for all success/failure paths without network access

Evidence

• delivery DAG: verification PASS, review ACCEPT; parent correction wave closed the missing-PR exit-0 path
• bash script/specgit-bootstrap.test.sh: 275 passed, 0 failed
• sh -n / bash -n / shellcheck: passed
• commit hook: root lint and full workspace typecheck passed

Checklist

• New deliveries target dev without a manual edit
• Exit 0 requires a unique PR with verified base dev
• Post-success failures retain the new binding for retry
• chore(specgit): automate bootstrap around required harness specializations #521/fix(specgit): restore delivery state after failed bootstrap #530/fix(specgit): reject title types unsupported by branch ruleset #529 contracts remain green
• GitHub default branch and rulesets remain unchanged