Expose automation origin metadata

[cbusillo]

Summary

• add structured automation origin metadata for externally launched sessions via CODE_AUTOMATION_ORIGIN
• persist origin metadata in rollout SessionMeta and include it in session_configured
• update app-server protocol schema fixtures/types and document the launcher contract

Validation

• cargo test -p code-core automation_origin -- --nocapture
• cargo test -p code-protocol serialize_session_configured_with_automation_origin -- --nocapture
• cargo run -p code-app-server-protocol --bin write_schema_fixtures
• cargo test -p code-app-server-protocol schema_fixtures_match_generated -- --nocapture
• git diff --check
• ./build-fast.sh

Refs #160
Refs #27