feat: semantic-search fallback in the search command

[mayank-d3]

Summary

agent-root search now falls back to the registry's hybrid semantic endpoint (/api/search, BM25 + pgvector RRF) when keyword search (/api/records) returns no results — so natural-language queries like "skill to pay in USDC" surface relevant records instead of nothing. Fallback hits are clearly labeled as approximate. Drafted with help from Claude; I reviewed every line before submitting.

Changes

• Zero-result fallback to /api/search, ordered: records → semantic → legacy (find-skills, manifest probe)
• New --no-semantic flag (keyword-only opt-out)
• Approximate hits shown under a // no exact match — closest semantic matches: header, each tagged {hybrid}/{vector}/{keyword}
• --json gains a top-level approximate boolean (uniform across empty + non-empty envelopes)
• One request per failed search (respects the 5-req/min rate limit); --all unaffected; embedder-offline surfaces naturally as {keyword} tags
• vitest tests, CHANGELOG, README, per-command help, and docs/TESTING.md updated

Checklist

• Conventional Commit message format (feat:, docs:)
• Added tests under tests/ that fail without this change (mappers, searchSemantic incl. 429 → [], fallback ordering, display + --json)
• pnpm test passes locally (only the pre-existing live-DNS integration-smoke tests fail in a no-network sandbox)
• Updated CHANGELOG.md under ## [Unreleased]
• Updated README.md + per-command help for the new flag/behavior
• Commits signed off with DCO (git commit -s)
• Manually verified — see "How to test"

How to test

Built locally (pnpm build) and run against production:

node dist/bin/agentroot.js search "skill to pay in USDC"                 # closest semantic matches + {vector} tags
node dist/bin/agentroot.js search "skill to pay in USDC" --no-semantic   # No records found
node dist/bin/agentroot.js search "payments"                            # normal keyword hit, no semantic header
node dist/bin/agentroot.js search "stablecoin checkout" --json | jq '{approximate, match: .results[0].match}'   # {"approximate":true,"match":"vector"}
node dist/bin/agentroot.js search "payments" --json | jq .approximate    # false

Closes / Related

Closes #1