feat: add interactive file path cards in chat UI

Render filepath code blocks as rich, clickable cards with three variants:
knowledge files (navigate to editor), audio files (inline play/pause),
and system files (open externally). Adds shell:openPath and
shell:readFileBase64 IPC channels, FileCardProvider context, and
Streamdown pre override.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/x/packages/core/src/application/assistant/instructions.ts

@@ -179,4 +179,23 @@ When a user asks for ANY task that might require external capabilities (web sear
 
 **Only \`executeCommand\` (shell/bash commands) goes through the approval flow.** If you need to delete a file, use the \`workspace-remove\` builtin tool, not \`executeCommand\` with \`rm\`. If you need to create a file, use \`workspace-writeFile\`, not \`executeCommand\` with \`touch\` or \`echo >\`.
 
-Rowboat's internal builtin tools never require approval — only shell commands via \`executeCommand\` do.`;
+Rowboat's internal builtin tools never require approval — only shell commands via \`executeCommand\` do.
+
+## File Path References
+
+When you reference a file path in your response (whether a knowledge base file or a file on the user's system), ALWAYS wrap it in a filepath code block:
+
+\`\`\`filepath
+knowledge/People/Sarah Chen.md
+\`\`\`
+
+\`\`\`filepath
+~/Desktop/report.pdf
+\`\`\`
+
+This renders as an interactive card in the UI. Use this format for:
+- Knowledge base file paths (knowledge/...)
+- Files on the user's machine (~/Desktop/..., /Users/..., etc.)
+- Audio files, images, documents, or any file reference
+
+Never output raw file paths in plain text when they could be wrapped in a filepath block.`;