feat: make rusty-claude-cli usable end-to-end

Wire the CLI to the Anthropic client, runtime conversation loop, and MVP in-tree tool executor so prompt mode and the default REPL both execute real turns instead of scaffold-only commands.

Constraint: Proxy auth uses ANTHROPIC_AUTH_TOKEN as the primary x-api-key source and may stream extra usage fields
Constraint: Must preserve existing scaffold commands while enabling real prompt and REPL flows
Rejected: Keep prompt mode on the old scaffold path | does not satisfy end-to-end CLI requirement
Rejected: Depend solely on raw SSE message_stop from proxy | proxy/event differences required tolerant parsing plus fallback handling
Confidence: medium
Scope-risk: moderate
Reversibility: clean
Directive: Keep prompt mode tool-free unless the one-shot path is explicitly expanded and reverified against the proxy
Tested: cargo test -p api; cargo test -p tools; cargo test -p runtime; cargo test -p rusty-claude-cli; cargo build; cargo run -p rusty-claude-cli -- prompt "say hello"; printf '/quit\n' | cargo run -p rusty-claude-cli --
Not-tested: Full interactive tool_use roundtrip against the proxy in REPL mode

rust/crates/api/src/error.rs

@@ -50,11 +50,14 @@ impl Display for ApiError {
             Self::MissingApiKey => {
                 write!(
                     f,
-                    "ANTHROPIC_API_KEY is not set; export it before calling the Anthropic API"
+                    "ANTHROPIC_AUTH_TOKEN or ANTHROPIC_API_KEY is not set; export one before calling the Anthropic API"
                 )
             }
             Self::InvalidApiKeyEnv(error) => {
-                write!(f, "failed to read ANTHROPIC_API_KEY: {error}")
+                write!(
+                    f,
+                    "failed to read ANTHROPIC_AUTH_TOKEN / ANTHROPIC_API_KEY: {error}"
+                )
             }
             Self::Http(error) => write!(f, "http error: {error}"),
             Self::Io(error) => write!(f, "io error: {error}"),