feat(retrieval): manifest-first bounded knapsack retrieval to reduce context consumption

[harshsinghmp]

Classification

actionable-feature

Context & Rationale

Injecting full memory notes into agent prompts burns significant context tokens and risks hallucinations. Inspired by Secretary's bounded retrieval, injecting a compact manifest allows agents to select only what is needed.

Key Capabilities & Scope

1. Manifest-First Injection:
   • Instead of injecting full note bodies on session start, inject a compact JSON/YAML manifest (id, hash, title, tags, one_line_summary).
2. On-Demand Hydration:
   • Provide a fast tool or CLI flag to hydrate and read the full note only when selected.
3. Knapsack Optimization:
   • Bound total injected manifest size to a strict token budget (<500 tokens).

Verification Criteria

• Knapsack retrieval returns manifest summaries by default.
• Specific entries can be hydrated on demand.
• Passes test suite (bun test test/).