feat: env-based PRIVATE_KEY + A2A URL support

[jamesyoung]

Summary

• Read PRIVATE_KEY from .env with validation in src/setup.ts
• sync.ts accepts A2A JSON URL, uses name as top-level key, includes present fields only
• search.ts accepts A2A JSON URL, uses name as search value
• README: motivation, requirements, env setup, A2A format, run instructions, troubleshooting

Motivation

• Standardize secrets via env
• A2A card–driven sync/search flow
• Improve discoverability after registering on Intuition

Test Plan

• printf "PRIVATE_KEY=0x\n" > .env
• pnpm i
• pnpm tsx src/search.ts https://example.com/a2a.json
• pnpm tsx src/sync.ts https://example.com/a2a.json (requires tTRUST)

Notes

• No breaking changes
• .env is gitignored