Add the remaining simple issue mutations. `issue delete` hard-deletes via
the DELETE endpoint and is deliberately not idempotent — a nonexistent
issue yields ISSUE_NOT_FOUND rather than reporting success. `issue pin`
and `issue unpin` read the current pin state (Gitea's pin_order field) and
short-circuit to an idempotent no-op with an Already pinned/unpinned
message when there is nothing to do.
- 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
