fix(discovery): replace require() with ESM import for viem in cache writer

[uibeka]

Problem

Agent discovery cache writes silently fail on every call. The discovered_agents_cache SQLite table is never populated, forcing agents to perform full on-chain Transfer event scans (2081+ events, ~6 seconds) on every discover_agents call. Logs show 20+ require is not defined errors per discovery call (one per agent card enrichment).

Root Cause

setCachedCard in src/registry/discovery.ts uses require("viem") to import keccak256 and toBytes for computing agent card hashes. The project is ESM ("type": "module" in package.json, all imports use .js extensions), so require() is not available. The error is caught by the surrounding try-catch and silently swallowed, making the failure invisible except in verbose logs.

Fix

Replace require("viem") with a standard ESM import. The keccak256 and toBytes functions are imported at the top of the file, consistent with how other modules in the codebase import from viem.

Impact

• Discovery cache writes succeed — discovered_agents_cache table populates correctly
• Subsequent discovery calls can use cached agent cards instead of re-fetching from the network
• Eliminates 20+ ReferenceError stack traces per discovery call from logs
• Zero behavioral change for agents — discovery results are identical, just cached for performance
• No new dependencies — viem is already a project dependency

Testing

• pnpm build — zero errors (import resolves correctly)
• pnpm test — all 1005 existing tests pass (27 test files)

Note: Related Issue (Not Fixed Here)

During investigation, we observed that agents using data: URIs for inline ERC-8004 agent cards are blocked by SSRF protection (isAllowedUri only allows https: and ipfs:). data: URIs don't make network requests and pose no SSRF risk, but they're a legitimate pattern for inline agent metadata. Happy to open a separate issue or PR if the team wants to support data: URIs for agent cards.