chore: remove bundled benchmark tooling

[vishr]

What changed

• Remove the bundled cmd/bench driver and its tests.
• Remove the obsolete in-repository benchmark documentation and performance benchmark test.
• Stop building bin/bench with the production binary.
• Point README and site documentation to the public, independent fanout-bench harness.

The standalone harness keeps benchmark dependencies, artifacts, and iteration outside the single-binary product repository.

Verification

• just check
• just test-race when auth, API, ingest, query, MCP, or agent paths changed — not applicable; no runtime path changed
• User-facing behavior and configuration docs are current
• No credentials, private telemetry, host details, or enterprise-only source are included
• API, migration, ingest, MCP/AG-UI, or release-contract changes are called out — none

Additional verification:

• go test ./... — 967 tests passed
• just site-build — 0 Astro diagnostics; 33 pages built
• labstack/fanout-bench visibility verified as public
• Local benchmark example supplies the dedicated private-metrics credential.

[vishr]

Addressed both review findings in c6129ad:

• Verified https://github.com/labstack/fanout-bench is publicly accessible.
• Corrected the local example to configure FANOUT_METRICS_TOKEN and pass the same value as FANOUT_BENCH_METRICS_TOKEN; metrics remain private by default.

Validation: just site-build and the full pre-push just check both pass.