Replace CONTEXT_DEFAULTS with prompt file frontmatter discovery
Move context metadata (tier, label, group) from hardcoded CONTEXT_DEFAULTS
dict to YAML frontmatter in each prompt .md file. This establishes a single
source of truth for context configuration.
Changes:
- Add frontmatter to 8 prompt files with context/tier/label/group
- Create apps/chat/title.md (replaces inline TITLE_SYSTEM_INSTRUCTION)
- Add PROMPT_PATHS list and _discover_prompt_contexts() in models.py
- Remove CONTEXT_DEFAULTS dict and observe.summarize (unused)
- Update tests to validate prompt frontmatter instead of static dict
- Fix stale comments referencing "static defaults"
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>