build(deps): depend on published @intx 0.4.0 (CL-8797)

[TheGreatAxios]

Summary

• switch @intx/agent and @intx/types from * peer / optional-peer entries to peerDependencies at >=0.4.0 and devDependencies at exact 0.4.0, matching the convention in corbitsdev/granola-tools and corbitsdev/corbits-credential-mcp
• delete scripts/link-intx.ts, the link:intx script, bunfig.toml's intx-src resolve condition, the --conditions intx-src test flags, and tsconfig.json's customConditions
• delete src/shims/intx-agent.d.ts and tsconfig.typecheck.json; typecheck now runs tsc --noEmit against the real published @intx/agent/@intx/types types — no type errors surfaced, so no source changes were needed
• drop the sibling ../interchange checkout from .github/workflows/test.yml; CI now just installs and runs against the published packages
• update README/AGENTS/ARCHITECTURE/CONTRIBUTING to drop the local-checkout instructions

Validation

A fresh clone with no sibling checkout works:

bunx bun@1.3.14 install --frozen-lockfile
bun run typecheck
bun run test
bun run build

All pass.

Part of CL-8797.