parse_proxy_config was using expect()/unwrap()/panic!() which would
crash the process on malformed proxy config strings instead of
returning a descriptive error. Now returns ConfigError::ProxyConfigInvalid
with the specific issue (bad scheme, bad address). Added tests for
invalid scheme, invalid address, and end-to-end from_serve_options.
Review found Phase 1 approved with minor issues. Created cleanup tasks:
- napi-identity-provider-wiring (bug: NAPI bypasses IdentityProvider)
- panic-free-static-config (code smell: panic/unwrap in production path)
- non-exhaustive-public-api (future-proofing public API types)
- adr-doc-comments (doc convention: ADR references in new modules)
- ssh-session-recv-stub-doc (documentation: mark stubs as planned)
