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>