docs: adopt Conventional Commits and land planning docs

- CONVENTIONAL-COMMITS.md: vendored spec (v1.0.0); CLAUDE.md requires
  agent-written commits to follow it
- tasks 0002-0020: remaining task breakdown of the gitea-axi spec
  (0019 reframed around the three-tier test taxonomy: unit,
  integration, end-to-end)
- spec, CONTEXT.md, and ADRs 0006/0007/0011: pending design
  refinements from the planning sessions

CLAUDE.md

@@ -0,0 +1,5 @@
+# gitea-axi — Agent Instructions
+
+## Commits
+
+Any commit message you write must follow the Conventional Commits specification as documented in [CONVENTIONAL-COMMITS.md](CONVENTIONAL-COMMITS.md).