feat(hh): graceful shutdown, crypt default theme, neutralize branding, share-prep

- Graceful shutdown: Ctrl+C quits in chat (interrupts PTY while driving),
  RAII TermGuard + panic hook + SIGTERM/SIGHUP always restore the terminal
- Default theme is now "crypt" (neutral monochrome); theme sigil mirrored in
  chat/roster/help so the pentagram only renders under the "church" theme
- Neutralize inverted-pentagram branding across CLI, scripts, docs, and Cargo
  metadata (kept only in themes/church.toml + the render-time placeholder)
- Rewrite root README around hack-house; add bootstrap.sh, SECURITY.md,
  CODE_OF_CONDUCT.md, CHANGELOG.md, and issue/PR templates
- .gitignore cleanup; stop tracking .venv

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,26 @@
+## Summary
+What does this PR change, and why?
+
+## Related issues
+Closes #
+
+## Type of change
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Refactor / cleanup
+- [ ] Documentation
+- [ ] Tests
+
+## Checklist
+- [ ] `cargo build` and `cargo test` pass (client)
+- [ ] `cargo fmt` and `cargo clippy` are clean
+- [ ] `pytest` passes (server)
+- [ ] I did not commit secrets, `.venv`, or build artifacts
+- [ ] I updated docs / `CHANGELOG.md` where relevant
+
+## Security / privacy impact
+Does this affect encryption, the zero-knowledge server model, authentication,
+or sandbox permissions? If so, explain. If not, write "none".
+
+## How to test
+Steps for a reviewer to verify the change.