feat: prepare Agent Search MCP 3.2.0

[lennney]

Description

Prepare Agent Search MCP 3.2.0 as a conservative minor release focused on free, token-efficient Agent search.

This branch adds and hardens:

• 16 unified adapters: 9 zero-key and 7 optional BYOK APIs
• explicit free-first, quality-escalation, paid-first, and hard no-spend routing
• project-owned DuckDuckGo Web -> HTML -> Lite fallback without Python/ddgs
• request budgets, provider cooldown, exact cache, evidence packets, and compact output
• stdio/HTTP security, cancellation, proxy redaction, and provider-family accounting
• real Bing News RSS retrieval with 24-hour/7-day/30-day filters and visible partial failures
• Node 18 Streamable HTTP Web Crypto compatibility
• lightweight pooled quality evaluation with auditable AI review labels
• experimental MCP 2026 work isolated under experiments/mcp-2026
• Node 18-compatible Hono 1.19.15 security backport resolution

No npm publish, GitHub tag/Release, MCP Registry update, or product-homepage deployment is included in this PR.

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that causes existing functionality to break)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)
• Build/CI change

How Has This Been Tested?

npm run build
npm run lint
npm test                       # 74 files, 747 passed, 2 explicit live-network skips
npm run capabilities:check
npm run benchmark:verify
npm run benchmark:quality:verify

The earlier exact tarball replay covered Windows/Linux Node 18.20.8, 20.20.2, and 22.23.1, but the HTTP/news fixes changed package source. That artifact is retained as historical evidence only. A publishable tarball must be regenerated from the final PR commit and replayed after CI passes.

This change does not repeat DDG/Sogou live calls. The DDG Web/HTML/Lite main chain is unchanged, and no public availability or search-quality percentage is claimed.

Checklist

• Branch is based on latest main
• Only target files changed (the release branch intentionally includes tracked HANDOVER/plan/evidence updates)
• npm run build passes
• npm test passes (all offline tests; live tests are explicitly gated)
• npx tsc --noEmit passes through the build gate
• CHANGELOG.md updated (if user-facing change)
• README updated (if API/CLI/config change)