fix(parser): skip external-session-import sentinel in agent_message (#221)

[delexw]

What changed

Codex v0.147.0 (openai/codex PR #36356, part of #36361/#35623) lets Codex sync further edits into an already-imported Claude/Cursor conversation by appending new turns to the existing thread, instead of only ever finishing an import once.

I checked the real upstream diff (openai/codex#36356) rather than guessing:

• Session-identity concern (the issue's main "open question"): the sync path appends directly into the same rollout file for the same thread id — it never writes a new file or a new session_meta. codex-trace's existing (mtime, size) cache invalidation in cache.rs already re-parses on any change to that file, so no duplicate/orphaned session entries can appear and discover.rs needs no change here.
• Real bug found instead: Codex inserts a synthetic agent_message event with the sentinel text <EXTERNAL SESSION IMPORTED> as the last message of an imported thread (see EXTERNAL_SESSION_IMPORTED_MARKER in codex-rs/external-agent-migration/src/sessions/export.rs). Previously this sentinel only ever trailed a finished import. With v0.147.0's sync, it can now sit mid-transcript, right before newly-synced turns, since the append logic filters it out of the appended suffix but leaves the original one in place. turn.rs rendered any non-empty agent_message text as a real message, so this sentinel would show up as a fake assistant reply sitting between two real turns.

Fix

Skip the exact sentinel string in handle_event_msg so it's never rendered as a message or picked up as a final_answer.

Tests

Added two tests in src-tauri/src/parser/turn.rs:

• v0147_external_session_imported_marker_is_not_rendered_as_agent_message — the marker is suppressed and doesn't become the turn's final answer.
• v0147_synced_turn_after_import_marker_parses_as_its_own_turn — a turn appended after the marker (simulating a sync) still parses correctly as its own turn.

Verification

• cargo test --lib --manifest-path src-tauri/Cargo.toml — 362 passed
• cargo clippy --manifest-path src-tauri/Cargo.toml -- -D warnings — clean
• cargo fmt --manifest-path src-tauri/Cargo.toml --check — clean
• npx vitest run — 148 passed
• npx tsc --noEmit — clean
• npx oxlint — clean (one pre-existing, unrelated warning in MarkdownRenderer.tsx)

Fixes #221