fix: resolve file card paths for ~/.rowboat/ files and restrict filepath blocks to existing files

- Resolve workspace-relative and /tmp paths in shell:openPath and shell:readFileBase64 IPC handlers
- Update assistant instructions to only use filepath blocks for files that already exist

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

apps/x/apps/main/src/ipc.ts

@@ -462,6 +462,9 @@ export function setupIpcHandlers() {
       let filePath = args.path;
       if (filePath.startsWith('~')) {
         filePath = path.join(os.homedir(), filePath.slice(1));
+      } else if (!path.isAbsolute(filePath)) {
+        // Workspace-relative path — resolve against ~/.rowboat/
+        filePath = path.join(os.homedir(), '.rowboat', filePath);
       }
       const error = await shell.openPath(filePath);
       return { error: error || undefined };
@@ -470,6 +473,9 @@ export function setupIpcHandlers() {
       let filePath = args.path;
       if (filePath.startsWith('~')) {
         filePath = path.join(os.homedir(), filePath.slice(1));
+      } else if (!path.isAbsolute(filePath)) {
+        // Workspace-relative path — resolve against ~/.rowboat/
+        filePath = path.join(os.homedir(), '.rowboat', filePath);
       }
       const stat = await fs.stat(filePath);
       if (stat.size > 10 * 1024 * 1024) {

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

@@ -195,9 +195,11 @@ knowledge/People/Sarah Chen.md
 ~/Desktop/report.pdf
 \`\`\`
 
-This renders as an interactive card in the UI. Use this format for:
+This renders as an interactive card in the UI that the user can click to open the file. 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.`;
+**IMPORTANT:** Only use filepath blocks for files that already exist. The card is clickable and opens the file, so it must point to a real file. If you are proposing a path for a file that hasn't been created yet (e.g., "Shall I save it at ~/Documents/report.pdf?"), use inline code (\`~/Documents/report.pdf\`) instead of a filepath block. Use the filepath block only after the file has been written/created successfully.
+
+Never output raw file paths in plain text when they could be wrapped in a filepath block — unless the file does not exist yet.`;