Authenticate Workers with Cloudflare Access and reset agent routing

[zackbart]

Summary

• add Cloudflare Access as a Worker-specific inbound auth provider, with human operator access and MCP-only service-token access
• update the Worker example, operator flow, doctor command, package exports, migration docs, and 0.21.0 release notes
• route unknown-address reads through one execute_code call while keeping known-address reads on call_tool
• replace the issue-specific benchmark stack with four pinned, deterministic cases for routing, provider semantics, pagination privacy, latency, tokens, and result forwarding
• retain the compatibility JSON copy until the supported-client matrix permits its removal

The benchmark cleanup removes 17,179 lines and adds 1,721 across the full release change.

Evidence

The final one-pass agent run passed 4/4 cases in 83.0 seconds. It used 449 model-visible MCP result tokens. The private pagination case read three pages and returned only the count.

npm run release:check passes:

• 2,724 tests passed, 40 skipped
• production dependency audit found no vulnerabilities
• package and Docker smoke passed at 169 files and 557,861 bytes

Closes #506

Supersedes #496. The 30-session comparative gate described there no longer exists. The replacement pins gpt-5.6-sol and is a small diagnostic benchmark, not a statistical release comparison.