docs: default caveman style for issues, PRs, and reviews

Document caveman-full as the collaboration voice in AGENTS and CONTRIBUTING,
and add Gitea issue/PR templates that pre-fill terse structure.

AGENTS.md

@@ -24,6 +24,23 @@ See [`CONTRIBUTING.md`](CONTRIBUTING.md) for the development workflow and TDD re
 
 ## Conventions
 
+### Caveman style (default for collaboration)
+
+**Issues, pull requests, and code reviews** use **caveman-full** by default:
+
+- Short sentences or fragments; drop articles and filler when clear
+- Lead with fact, verdict, or ask — not preamble
+- Bullets over paragraphs; one idea per line
+- Keep technical terms, acronyms, paths, and code exact
+- English only; no identifying references (same as all repo prose)
+
+README tiers: root [`README.md`](README.md) = **caveman-lite** (full sentences, less filler);
+component READMEs = **caveman-full**. Design docs in `docs/` stay readable prose unless
+editing for consistency.
+
+Templates: [`.github/ISSUE_TEMPLATE/`](.github/ISSUE_TEMPLATE/),
+[`.github/pull_request_template.md`](.github/pull_request_template.md).
+
 - Markdown: ATX headings, fenced code blocks with language tags, pipe tables.
 - Python: PEP 8, type hints, pure functions where possible, no unnecessary classes.
 - TypeScript: strict mode, functional components and hooks, no extra UI libraries.