[codex-analytics] add extensible feature thread sources

[marksteinbrick-oai]

Why

• ThreadSource currently defines a closed set of core-owned values
• Product features also create threads for background or scheduled work
• Adding every product-specific value to the core enum would require repeated codex-rs protocol changes
• Feature-backed values let product callers provide precise attribution while preserving the existing core classifications

What Changed

• Adds ThreadSource::Feature(String) for app-owned thread source values
• Represents all app-server v2 thread sources as scalar strings, so a feature source is supplied as "automation"
• Persists and emits the feature's plain string label, so "automation" produces thread_source="automation" in analytics
• Keeps user, subagent, and memory_consolidation as explicit core-owned values and regenerates the app-server schemas and TypeScript bindings

Verification

• just write-app-server-schema
• cargo check --workspace
• just test -p codex-protocol feature_thread_source_serializes_as_its_app_owned_label
• just test -p codex-app-server-protocol thread_sources_round_trip_as_scalar_labels
• cargo test -p codex-analytics thread_initialized_event_serializes_expected_shape
• just fmt

[chatgpt-codex-connector]

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