feat: Add context-hub haystack connector

[AlexMikhalev]

Summary

Add a new haystack connector for context-hub (Andrew Ng's curated API docs for coding agents) to complement the existing grepapp_haystack and query_rs haystacks.

Motivation

Terraphim's haystack architecture already covers two code knowledge sources:

Source	Status	What it searches
grep.app	Implemented (grepapp_haystack)	Code across ~1M GitHub repos -- "How do others use this API?"
query.rs	Implemented (query_rs haystack)	Rust std lib, crates, type signatures, error codes, Clippy lints, books -- "What is the Rust API for X?"
context-hub	Proposed	Curated, versioned API docs (Stripe, OpenAI, etc.) with agent annotations -- "Give the agent correct, hallucination-free API docs"

Context-hub fills a gap neither grep.app nor query.rs covers: curated, language-aware API documentation specifically designed for AI coding agents. It provides incremental doc fetch (token-efficient), community-maintained quality, language variants (Python/JS), and persistent annotations.

How context-hub differs from existing haystacks

Dimension	grep.app	query.rs	context-hub
Data	Source code	Rust docs, types, lints	Curated API docs (multi-language)
Query model	Text/regex	Prefixed search + type signatures	Hierarchical IDs + chub search
Freshness	Live index of GitHub	Synced with Rust releases	Community-maintained PRs
Privacy	Public repos only	Public docs	Public repo
Best for	Usage examples	Rust API reference, error lookup	Agent-friendly API context for any language
Stack	Web API	Web API (scraping + JSON)	CLI (chub) + markdown repo

Proposed Design

chub_haystack

• Implement IndexMiddleware trait (same pattern as grepapp and query_rs haystacks)
• Wrap chub search and chub get CLI or replicate the search/fetch protocol directly via HTTP
• Map context-hub markdown + YAML frontmatter to Terraphim Document types
• Preserve language variants (Python/JS) as document metadata
• Optionally bridge annotations back to Terraphim's learning capture system
• Feature-gate behind context-hub flag in terraphim_middleware

Acceptance Criteria

• chub_haystack module in crates/terraphim_middleware/src/haystack/
• Implements IndexMiddleware trait
• Feature flag context-hub in middleware Cargo.toml
• Integration tests with real queries (not mocks)
• Config support in role configs (similar to grepapp/query_rs entries)