fix(mimicry): align prompt-caching, context-management, and cache scope with CC 2.1.90

Three mimicry gaps closed:
- prompt-caching-scope now always-on for firstParty (was non-interactive only)
- context-management now model-gated for Claude 4+ (was env-var gated)
- cache_control includes scope: "global" field matching upstream CC

Constraint: redact-thinking intentionally excluded — user prefers seeing thinking blocks
Rejected: env-var gating for context-management | upstream CC uses model detection
Confidence: high
Scope-risk: narrow