Replace bespoke CLI line editing with rustyline and canonical model aliases

The REPL now wraps rustyline::Editor instead of maintaining a custom raw-mode
input stack. This preserves the existing LineEditor surface while delegating
history, completion, and interactive editing to a maintained library. The CLI
argument parser and /model command path also normalize shorthand model names to
our current canonical Anthropic identifiers.

Constraint: User requested rustyline 15 specifically for the CLI editor rewrite
Constraint: Existing LineEditor constructor and read_line API had to remain stable
Rejected: Keep extending the crossterm-based editor | custom key handling and history logic were redundant with rustyline
Rejected: Resolve aliases only for --model flags | /model would still diverge from CLI startup behavior
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep model alias normalization centralized in main.rs so CLI flag parsing and /model stay in sync
Tested: cargo check --workspace
Tested: cargo test --workspace
Tested: cargo build --workspace
Tested: cargo clippy --workspace --all-targets -- -D warnings
Not-tested: Interactive manual terminal validation of Shift+Enter behavior across terminal emulators

rust/crates/api/src/client.rs

@@ -520,6 +520,7 @@ fn read_auth_token() -> Option<String> {
         .and_then(std::convert::identity)
 }
 
+#[must_use]
 pub fn read_base_url() -> String {
     std::env::var("ANTHROPIC_BASE_URL").unwrap_or_else(|_| DEFAULT_BASE_URL.to_string())
 }