feat: Rust port of Claude Code CLI

Crates:
- api: Anthropic Messages API client with SSE streaming
- tools: Claude-compatible tool implementations (Bash, Read, Write, Edit, Glob, Grep + extended suite)
- runtime: conversation loop, session persistence, permissions, system prompt builder
- rusty-claude-cli: terminal UI with markdown rendering, syntax highlighting, spinners
- commands: subcommand definitions
- compat-harness: upstream TS parity verification

All crates pass cargo fmt/clippy/test.

rust/crates/api/src/error.rs

@@ -0,0 +1,65 @@
+use std::env::VarError;
+use std::fmt::{Display, Formatter};
+
+#[derive(Debug)]
+pub enum ApiError {
+    MissingApiKey,
+    InvalidApiKeyEnv(VarError),
+    Http(reqwest::Error),
+    Io(std::io::Error),
+    Json(serde_json::Error),
+    UnexpectedStatus {
+        status: reqwest::StatusCode,
+        body: String,
+    },
+    InvalidSseFrame(&'static str),
+}
+
+impl Display for ApiError {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::MissingApiKey => {
+                write!(
+                    f,
+                    "ANTHROPIC_API_KEY is not set; export it before calling the Anthropic API"
+                )
+            }
+            Self::InvalidApiKeyEnv(error) => {
+                write!(f, "failed to read ANTHROPIC_API_KEY: {error}")
+            }
+            Self::Http(error) => write!(f, "http error: {error}"),
+            Self::Io(error) => write!(f, "io error: {error}"),
+            Self::Json(error) => write!(f, "json error: {error}"),
+            Self::UnexpectedStatus { status, body } => {
+                write!(f, "anthropic api returned {status}: {body}")
+            }
+            Self::InvalidSseFrame(message) => write!(f, "invalid sse frame: {message}"),
+        }
+    }
+}
+
+impl std::error::Error for ApiError {}
+
+impl From<reqwest::Error> for ApiError {
+    fn from(value: reqwest::Error) -> Self {
+        Self::Http(value)
+    }
+}
+
+impl From<std::io::Error> for ApiError {
+    fn from(value: std::io::Error) -> Self {
+        Self::Io(value)
+    }
+}
+
+impl From<serde_json::Error> for ApiError {
+    fn from(value: serde_json::Error) -> Self {
+        Self::Json(value)
+    }
+}
+
+impl From<VarError> for ApiError {
+    fn from(value: VarError) -> Self {
+        Self::InvalidApiKeyEnv(value)
+    }
+}