Lock down CLI-to-mock behavioral parity for Anthropic flows

This adds a deterministic mock Anthropic-compatible /v1/messages service,
a clean-environment CLI harness, and repo docs so the first parity
milestone can be validated without live network dependencies.

Constraint: First milestone must prove Rust claw can connect from a clean environment and cover streaming, tool assembly, and permission/tool flow
Constraint: No new third-party dependencies; reuse the existing Rust workspace stack
Rejected: Record/replay live Anthropic traffic | nondeterministic and unsuitable for repeatable CI coverage
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep scenario markers and expected tool payload shapes synchronized between the mock service and the harness tests
Tested: cargo fmt --all
Tested: cargo clippy --workspace --all-targets -- -D warnings
Tested: cargo test --workspace
Tested: ./scripts/run_mock_parity_harness.sh
Not-tested: Live Anthropic responses beyond the five scripted harness scenarios

rust/crates/mock-anthropic-service/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "mock-anthropic-service"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+publish.workspace = true
+
+[[bin]]
+name = "mock-anthropic-service"
+path = "src/main.rs"
+
+[dependencies]
+api = { path = "../api" }
+serde_json.workspace = true
+tokio = { version = "1", features = ["io-util", "macros", "net", "rt-multi-thread", "signal", "sync"] }
+
+[lints]
+workspace = true