feat(polling): add custom prompt support for Gmail email polling (#471)

Co-authored-by: Letta Code <noreply@letta.com>

docs/configuration.md

@@ -728,7 +728,58 @@ polling:
 | `polling.intervalMs` | number | `60000` | Polling interval in milliseconds |
 | `polling.gmail.enabled` | boolean | auto | Enable Gmail polling. Auto-detected from `account` or `accounts` |
 | `polling.gmail.account` | string | - | Gmail account to poll for unread messages |
-| `polling.gmail.accounts` | string[] | - | Gmail accounts to poll for unread messages |
+| `polling.gmail.accounts` | (string \| object)[] | - | Gmail accounts to poll. Can be strings or objects with `account`, `prompt`, `promptFile` |
+| `polling.gmail.prompt` | string | - | Default custom prompt for all accounts |
+| `polling.gmail.promptFile` | string | - | Path to default prompt file for all accounts |
+
+### Custom Email Prompts
+
+You can customize what the agent is told when new emails are detected. The custom text replaces the default body while keeping the silent mode envelope (account, time, trigger metadata, and messaging instructions).
+
+**Inline prompt for all accounts:**
+
+```yaml
+polling:
+  gmail:
+    enabled: true
+    prompt: "Summarize these emails and flag anything urgent."
+    accounts:
+      - user@example.com
+```
+
+**Per-account prompts:**
+
+```yaml
+polling:
+  gmail:
+    enabled: true
+    prompt: "Review these emails and notify me of anything important."
+    accounts:
+      - "personal@gmail.com"           # Uses global prompt above
+      - account: "work@company.com"
+        prompt: "Focus on emails from executives and flag urgent matters."
+      - account: "notifications@example.com"
+        promptFile: ./prompts/notifications.txt  # Re-read each poll
+```
+
+**Prompt file for live editing:**
+
+```yaml
+polling:
+  gmail:
+    enabled: true
+    promptFile: ./prompts/email-review.md  # Re-read each poll for live editing
+    accounts:
+      - user@example.com
+```
+
+**Priority order:**
+
+1. Account-specific `prompt` (inline in accounts array)
+2. Account-specific `promptFile`
+3. Global `polling.gmail.prompt`
+4. Global `polling.gmail.promptFile`
+5. Built-in default prompt
 
 ### Legacy config path