chore: migrate AgencyProxy to endpoint-libs 3

[pathscale]

Outcome

Moves the complete AgencyProxy workspace to published endpoint-libs 3.0.0 and releases the protocol, client, and binary as 0.1.8. The wire format is unchanged; the source changes only convert owned JSON strings into the new immutable UTF-8 byte wrapper.

Reliability

Replaces the drain test fake provider sleep with an explicit release sentinel. The old two-second assertion consumed one second inside the fixture and failed under parallel load even when shutdown was correct.

Supply chain

Pins checkout, Rust toolchain, and Rust cache Actions to immutable commits in CI and publish workflows. The application Cargo.lock remains tracked and now resolves the registry copy of endpoint-libs 3.0.0.

Verification

• Resolved endpoint-libs 3.0.0 from crates.io with registry checksum
• Strict workspace Clippy with all targets and features
• Complete workspace suite: 20 tests passed
• Deterministic drain test: 20 consecutive isolated passes
• cargo audit: zero vulnerabilities; two allowed transitive maintenance warnings
• agency-proxy-protocol 0.1.8 package verification passed
• Previously validated the same migration through a full AgencyZero debug build, live proxy sidecar, inspection interaction, and graceful shutdown