refactor(sandcastle): generalize actor/critic loop with LoopContext and coherent API

refactor(sandcastle): generalize actor/critic loop with LoopContext and coherent API

- Introduce LoopContext to group invariant params (spec, sandbox, strategy, baseBranch, signal)
- Extract GIT_BASE_BRANCH constant, thread through loop + finalization + prompts
- Rename 'implementer' to 'actor' in generic loop code
- Add extensibility: validate?, actorModel?, criticModel? on LoopStrategy
- Add baseBranch to RefinementLoopOptions and LoopResult
- Make Finding.category extensible (z.string())
- Make TaskSpec.labels optional
- Fix param ordering: pushBranch(spec, cwd, ...), deduplicateFindings(findings, cwd, ...)
- Simplify root lint-staged to cover all TS files