feat(skill): skill search via BM25 index for large skill catalogues

[claudianus]

Problem

When a user has a large number of skills installed (e.g. 1,000+), the current implementation injects the entire skill listing into the system prompt — including name, description (250 chars), whenToUse, and file path for every skill.

With 1,530 real skills this amounts to ~118,000 tokens in the system prompt alone, which:

1. Consumes ~46% of a 256K context window
2. Loads all SKILL.md file bodies into memory at startup (~88 MB JS heap)
3. Forces the LLM to scan a massive listing to find the right skill — poor accuracy due to lost-in-the-middle effects

Proposed Solution

A 3-tier auto-detection system based on invocable skill count:

Skill count	Listing format	Strategy
≤ 80	Legacy full listing (name + desc + path)	Prompt-cache optimal for small catalogues
81–300	Compact (name + 80-char desc)	Search available but not required
> 300	Names only	Search required via action: "search"

Key changes:

• BM25 search index built at startup from skill metadata (name, description, whenToUse). Zero external dependencies, ~24ms for 1,500 skills, <1ms per query.
• Skill tool extended with action: "search" (new) alongside existing action: "load" (default). Backward-compatible — skill field remains required.
• Lazy content loading: only frontmatter parsed at startup; full SKILL.md body loaded from disk on-demand when a skill is activated (~0.4ms per activation).
• Auto-detection: no feature flag needed — tier selected by skill count at session start.

Token savings (measured with real 1,530 skills)

Metric	Before	After
System prompt	118,120 tokens	8,436 tokens (93% reduction)
Startup memory	~88 MB	~4 MB (95% reduction)
Search latency	N/A	0.0–0.2ms

Files changed

• skill/search.ts (new) — BM25 index with synonym expansion
• skill/registry.ts — auto-detect tier, lazy load, searchSkills()
• skill/parser.ts — parseSkillMetaFromFile() for frontmatter-only parsing
• skill-tool.ts / .md — search action on existing Skill tool
• system.md — search-first workflow instructions
• Tests merged into existing registry.test.ts + integration proof in integration-proof.test.ts