fix(acp): align protocol version negotiation

[benbrandt]

Summary

• Align standalone v2 agents and agent routers with the v2 selection rule and structured -32022 mismatch error.
• Preserve native v1 legacy fallback with or without the draft v2 feature enabled.
• Share v1 initialization normalization across standalone and routed agents without enabling file-system or terminal capabilities from unknown newer fields.
• Update conductor rejection handling and assertions while retaining exact-version proxy initialization.
• Cover negotiation, legacy-client error handling, rejection flushing under backpressure, and no routed application startup after rejection.
• Expand routing guidance and add the no-default-features negotiation matrix to just test.

Protocol companion: agentclientprotocol/agent-client-protocol#2412

Compatibility

No blanket v1/v2 traffic conversion or fixed-response initializer is introduced. V1 clients retain the v2-only diagnostic as an unknown numeric JSON-RPC error.

The runtime helper works against the pinned schema dependency. No schema dependency bump or v1 error-catalog change is required.

Validation

• just test (all-features workspace and core without default features)
• cargo fmt --all -- --check
• cargo clippy -p agent-client-protocol --all-targets --all-features --locked -- -D warnings
• cargo clippy -p agent-client-protocol --all-targets --no-default-features --locked -- -D warnings
• mdbook build
• git diff --check