feat(FN-1170): add instruction bundle support to AgentStore
- Add InstructionsBundleConfig to core agent types, inputs, snapshots, diffing, and package exports
- Extend AgentStore persistence and config snapshot cloning to include bundleConfig state
- Implement managed bundle directory APIs for listing, reading, writing, deleting, and validating markdown instruction files
- Add migrateLegacyInstructions to convert legacy instructionsText/instructionsPath data into managed bundle config
- Add comprehensive AgentStore tests for bundle CRUD, validation rules, config updates, and legacy migration behavior
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