feat(signals): serve the live Venus health factor by default

[fercodes]

Sat 5 (stream S): serve the live Venus health factor by default — team funded the watch wallet, the demo position was created on Venus BSC testnet, and GET /agents/hf-watch/signal now reads real on-chain state (fixtures as fallback only).

Summary

• Watch wallet funded (0.0471 tBNB); demo position created: 0.035 tBNB minted into vBNB, borrowed 8 $U (vU market) after finding the testnet vBNB borrow cap at 0 and vWBNB borrow paused — health factor 1.838, liq price $326, live
• Watch address baked as the reader default (VENUS_WATCH_ADDRESS still overrides)
• Evidence DIY leg re-measured against the real position; agent leg of the TermiX report stays pending on the marketplace hire
• 4 on-chain txs recorded in openspec/changes/use-venus-testnet-position/proposal.md

Workstream

• P Platform
• C Catalog
• H Commerce
• A Agents
• S Signal

Review first

• packages/signals/src/index.ts — one-line default-constant addition; no behavior change when env unset previously was fixture (now serves live with fixture fallback on any reader failure)
• Provenance refresh in packages/signals/README.md (hf-watch metrics: live/ACTIVE)
• Testnet protocol constraints documented (vBNB borrow cap 0, vWBNB pause) — that is why the borrow side uses $U

Out of scope

• No other stream's tree; no domain changes; Pancake APR still deferred; marketplace-of-record agent leg (evidence) pending A/C.

Test plan

• pnpm -r typecheck — green across all 6 packages
• Live signal verified end-to-end: API serving HF 1.838/$326 (curl on both cache paths), reader standalone pass, fallback path intact for other agents
• On-chain: mint/enter/borrow txs on testnet.bscscan.com (hashes in the proposal)