ci: add sandcastle autonomous agent pipeline

ci: add sandcastle autonomous agent pipeline

Automated issue→PR workflow using Docker-sandboxed AI agents with an
implement↔critic refinement loop. Includes:

- GitHub issue discovery with planner agent
- Concurrent task execution with AbortSignal-based cooperative timeout
- Quality ratchet with content-addressed dedup and rollback
- Configurable strategy (prompts, validation, finalization)

Also simplifies clone-count workflow to use vars.GIST_ID instead of
auto-provisioned secret, and adds cspell words for sandcastle domain.