fix(specgit): reject unsupported branch title types

[LeXwDeX]

Closes #529

Why

SpecGit accepts conventional title types that this repository's branch rules reject. A bootstrap could therefore create remote issue/branch/PR state before GitHub rejected the derived branch prefix.

What changed

• preflight every issue-title operand against the eight AGENTS.md branch types before snapshot/init/inner CLI execution
• preserve numeric issue reuse and SpecGit 1.10.1 option grammar without rewriting accepted types
• reject unsupported types with exit 2 and stderr-only corrective diagnostics
• add red/green coverage for invalid types, all allowed types, multiple targets, numeric refs, options, --, help, and byte-exact passthrough

Evidence

• delivery DAG reviews: standards APPROVE, intent APPROVE, verification PASS, recovered arbiter ACCEPT
• bash script/specgit-bootstrap.test.sh: 202 passed, 0 failed
• sh -n script/specgit-bootstrap.sh
• bash -n script/specgit-bootstrap.test.sh
• shellcheck script/specgit-bootstrap.sh script/specgit-bootstrap.test.sh
• commit hook: root lint and full workspace typecheck passed

Checklist

• Unsupported types fail before init and forge side effects
• Allowed types are retained byte-for-byte; no silent mapping
• chore(specgit): automate bootstrap around required harness specializations #521 snapshot/restore and fix(specgit): restore delivery state after failed bootstrap #530 record rollback remain green
• fix(specgit): target delivery pull requests at dev #528 PR-base targeting remains out of scope