feat: anti-bypass base-pinning, diff-aware regression gates, neutral entrypoint

[renezander030]

Three enforcement gaps, mined from convergence across independent DoD tools (cogpin, donegate, god-mode-os, agent-guardrails) that all built anti-bypass mechanisms skillgate lacked.

What

• Base-pinning + fail-closed — check --pin / gate --pin read the spec from the git merge-base, not the working tree, so a change cannot loosen or delete the gate it is judged by. No resolvable base/spec => block, never a silent pass.
• Diff-aware regression gates — no-new (a pattern's match count must not increase vs base: skips, eslint-disable, TODOs) and no-deleted (files present at base must still exist). Deterministic; fail closed without a base ref.
• Neutral entrypoint — skillgate gate returns an allow/block decision for one command (raw or a hook JSON payload on stdin) so any harness (Cursor, Codex, git hook, shell) can enforce, not just Claude Code. Exit 0 allow / 2 block.

New/changed

• New src/git.ts (read-only git plumbing + dependency-free glob matcher), test/antibypass.test.ts, test/e2e-antibypass.test.ts
• src/core.ts, src/spec.ts, src/cli.ts, schema/done.schema.json

Verification

• 58 tests pass (+19). Coverage 94% lines / 82% branches / 98% functions — above the repo's 85/70/90 DoD gate.
• Smoke-tested against the built binary: an agent that strips a gate and adds a skip passes unpinned, is caught pinned.