feat: skill/prompt frontmatter now supports model, model-group, and thinking

[ofriw]

Skill/prompt frontmatter model selection

Skills and prompt templates can now declare model, model-group, and thinking alongside readonly. Interactive invocations apply model selection during input preflight, before slash-command expansion. readonly remains deferred to before_agent_start, where final skill metadata is available.

What changed

• Generalized the readonly-only cache into frontmatter-cache.ts, which parses and validates readonly, model-group, model, and thinking for skills and prompts.
• Added input preflight for model selection:
  1. model: provider/model-id selects that configured, authenticated model.
  2. Otherwise model-group: group-name resolves through the existing Model Group router.
  3. thinking: overrides the selected/current model’s level after capability clamping.
• Explicit model takes precedence over model-group and shows a warning when both are present.
• Unknown/unusable models or groups visibly block command expansion. Invalid frontmatter shows a warning and leaves valid independent fields usable.
• Model-selection frontmatter is ignored in headless sessions and blocked during streaming; no model/thinking mutation occurs.
• Preserved deferred readonly behavior and renamed internal readonly cache state/types to frontmatter cache equivalents.
• Added integration coverage for prompt/skill preflight, precedence, validation, auth and routing failures, thinking clamping, streaming, headless mode, cache freshness, and state reset.

Documentation

docs/architecture.md now documents the frontmatter fields, resolution order, and lifecycle split.

Verification

• npm run typecheck
• npm test (the existing audit-config check currently fails because npm audit reports an additional transitive advisory path; this PR does not change dependencies, the lockfile, or that test)