FM Phase 3b: Composable PayloadBuilder + recipes #24

Refactor Phase 3 assembly as a source pipeline instead of hard-coded sequence.

• PayloadSource interface (name, priority, produce, estimate_tokens)
• PayloadBuilder runs sources (parallel where safe), applies budget + shrinking
• Each current payload item becomes a source subclass: persona, dynamic_system, raw_turns, turn_summaries_recent, turn_summaries_search, session_summary, project_summary, rag, decisions, tools, new_user_message
• Recipes in config: [recipe.default], [recipe.lean], [recipe.research], [recipe.fresh_subject] — named ordered lists with per-source budget caps
• Agents override via X-Ragger-Recipe header or body ragger_overrides

Files: new include/ragger/payload_builder.h, new src/payload_builder.cpp, chat_sessions.cpp