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.

CONTRIBUTING.md

@@ -77,6 +77,41 @@ test: cover broadcast frame roundtrip
 Never include AI tool references, `Co-authored-by` trailers for assistants, or
 identifying names of people, companies, or locations.
 
+## Caveman style (default)
+
+Use **caveman-full** in **issues**, **pull requests**, and **reviews** unless the
+author explicitly asks otherwise.
+
+| Do | Skip |
+| --- | --- |
+| Short lines, fragments OK | Long preamble ("I wanted to start by saying…") |
+| Lead with what / why / verdict | Filler ("just", "basically", "kind of") |
+| Bullets, checklists | Repeated context already in diff or issue |
+| Exact terms, paths, code | Invented abbreviations or vague "the thing" |
+
+**Examples**
+
+| Fluffy | Caveman |
+| --- | --- |
+| I think we should probably consider adding a link checker to CI because links might break. | Add CI link checker. Broken relative links fail merge. |
+| This pull request implements the documentation improvements that were discussed in the audit. | Docs audit: glossary T0–T4, fix ground README link, rsync wording. |
+| LGTM but maybe we could think about whether the anchor format is consistent? | Merge OK. Fix glossary anchors in follow-up (#1). |
+
+README intensity: root = caveman-lite; `simulator/`, `prototype/`, `infra/`, `docs/adr/` READMEs =
+caveman-full. Numbered design docs in `docs/` stay full prose.
+
+Gitea pre-fills [issue](.github/ISSUE_TEMPLATE/default.md) and
+[PR](.github/pull_request_template.md) templates with this style.
+
+### Reviews
+
+Default review voice = caveman-full:
+
+1. **Verdict first** — merge / wait / no, or approve / request changes
+2. **Findings** — bullet per blocker or nit; file + line when useful
+3. **Scope** — say if issue AC met, partial, or out of scope
+4. No essay; link ADR, issue, or doc section instead of restating design
+
 ## Pull request checklist
 
 - [ ] Tests added or updated; `pytest tests/ -q` passes locally