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.

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,8 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Design docs
+    url: https://git.produktor.io/eSlider/swarm-house/src/branch/main/docs/12-design-journey.md
+    about: Start with the design journey, then open an issue here.
+  - name: Open questions
+    url: https://git.produktor.io/eSlider/swarm-house/src/branch/main/docs/09-open-questions.md
+    about: Known unknowns — check before duplicating.

.github/ISSUE_TEMPLATE/default.md

@@ -0,0 +1,24 @@
+---
+name: Change / task
+about: Default issue template — caveman style
+title: ""
+labels: []
+---
+
+Write **caveman-full**: short lines, fragments OK, no filler. See [`CONTRIBUTING.md`](../../CONTRIBUTING.md#caveman-style-default).
+
+## What
+
+<!-- One line. What need fix or build. -->
+
+## Why
+
+<!-- Problem or goal. Skip if obvious from title. -->
+
+## Done when
+
+- [ ]
+
+## Notes
+
+<!-- Links, ADR, open question §. Optional. -->