fix: replace panics with user-friendly errors in CLI session builder

[jeffa-block]

Summary

resolve_provider_and_model() and resolve_session_id() used .expect() which causes a raw panic with a full Rust stack trace when no provider or model is configured. Users see a frightening backtrace instead of a helpful message.

Changes

Replace all 7 .expect() calls in builder.rs with .unwrap_or_else() that prints a styled error via output::render_error() and exits cleanly. This matches the existing error handling pattern already used in the same file (e.g. ModelConfig::new() on line 390).

Before

thread 'main' panicked at crates/goose-cli/src/session/builder.rs:362:10:
No provider configured. Run 'goose configure' first
stack backtrace:
   0: std::sys::backtrace::...
   ...

After

  error: No provider configured. Run 'goose configure' first.

Converted calls

1. Missing provider name → styled error + exit
2. Missing model name → styled error + exit
3. current_dir() failure → styled error + exit
4. Session creation failure → styled error + exit
5. Working directory resolution → styled error + exit
6. User input failure → styled error + exit
7. System prompt file read failure → styled error with filename + exit

Fixes #7889

[DOsinga]

LGTM