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/tasks/0006-issue-delete-pin-unpin.md

@@ -0,0 +1,19 @@
+---
+spec: gitea-axi
+blocked-by: 0004-issue-create-and-comment
+---
+
+## What to build
+
+The remaining simple issue mutations: `issue delete`, `issue pin`, `issue unpin`.
+`issue delete <n>` hard-deletes via the DELETE endpoint (requires admin or owner permissions) and is deliberately not idempotent: a nonexistent issue errors with `ISSUE_NOT_FOUND` rather than reporting success (see ADR 0010).
+`issue pin <n>` and `issue unpin <n>` call the pin endpoints and are idempotent, returning early with `Already pinned`/`Already unpinned` messages.
+`issue lock`/`unlock`, `issue transfer`, and `issue subissue` remain excluded per the spec.
+
+## Acceptance criteria
+
+- [ ] `issue delete <n>` outputs `issue: { number, status: "deleted" }` on success
+- [ ] Deleting a nonexistent issue yields `ISSUE_NOT_FOUND` (exit 1), not idempotent success
+- [ ] `issue pin <n>` outputs `issue: { number, state, pinned }`; pinning an already-pinned issue returns early with `message: "Already pinned"` and exit 0
+- [ ] `issue unpin <n>` mirrors pin with `message: "Already unpinned"` on the no-op
+- [ ] Fixture-server tests cover delete success, delete-missing refusal, and both pin/unpin no-ops