[codex] Guard markdown widget file handling

[samuelstroschein]

Summary

• prevent non-markdown files from mounting the markdown editor
• show an unsupported-file placeholder that asks users to open an issue on GitHub
• fail closed when syncing the active markdown file id if a file widget lacks a markdown path
• add focused coverage for markdown path detection, unsupported files, and file widget opening behavior

Validation

• pnpm vitest run src/widget-runtime/widget-instance-helpers.test.ts
• pnpm exec prettier --check src/lib/path.ts src/widget-runtime/widget-instance-helpers.ts src/widget-runtime/widget-instance-helpers.test.ts src/widgets/markdown/index.tsx src/shell/layout-shell.tsx src/widgets/files/index.tsx src/widgets/markdown/index.test.tsx src/widgets/files/index.test.tsx

Notes

• The broader widget test suite and typecheck are blocked in this worktree by missing generated/submodule artifacts and SDK dependencies, including @markdown-wc/wasm and Lix SDK packages.

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Note

Low Risk
Defensive UI and key-value sync changes with fail-closed behavior; low risk aside from users no longer having non-markdown files treated as the active markdown document.

Overview
Adds shared isMarkdownFilePath detection and uses it so only markdown paths get the TipTap editor and flashtype_active_file_id updates.

Non-markdown file tabs still open in the central panel but show an unsupported-file placeholder (no editor mount, no raw content shown) so binary/CSV formatting is not corrupted. The shell derives the active markdown file via activeMarkdownFileIdFromWidgetInstance, which requires a markdown filePath and ignores other file widgets. Files drag-and-drop and copy now reuse the same path check; GitHub issue links point at the general issues page.

Tests cover path helpers, placeholder/sync behavior, and opening CSV as a file widget tab.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	flashtype-website	fd1ddda	Commit Preview URL Branch Preview URL	Jun 12 2026, 01:14 AM