feat: Persist agent prompt in DB so getAgentPrompt survives log cleanup

The `getAgentPrompt` tRPC procedure previously read exclusively from
`.cw/agent-logs/<name>/PROMPT.md`. Once the cleanup-manager removes
that directory, the prompt is gone forever.

Adds a `prompt` text column to the `agents` table and writes the fully
assembled prompt (including workspace layout, inter-agent comms, and
preview sections) to the DB in the same `repository.update()` call
that saves pid/outputFilePath after spawn.

`getAgentPrompt` now reads from DB first (`agent.prompt`) and falls
back to the filesystem only for agents spawned before this change.

Addresses review comment [MMcmVlEK16bBfkJuXvG6h].

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

apps/server/trpc/routers/agent.test.ts

@@ -50,6 +50,7 @@ function makeAgentInfo(overrides: Record<string, unknown> = {}) {
     updatedAt: new Date('2026-01-01T00:00:00Z'),
     userDismissedAt: null,
     exitCode: null,
+    prompt: null,
     ...overrides,
   };
 }
@@ -273,7 +274,7 @@ describe('getAgentPrompt', () => {
     await fs.writeFile(path.join(promptDir, 'PROMPT.md'), promptContent);
 
     const mockManager = {
-      get: vi.fn().mockResolvedValue(makeAgentInfo({ name: agentName })),
+      get: vi.fn().mockResolvedValue(makeAgentInfo({ name: agentName, prompt: null })),
     };
 
     const ctx = createTestContext({
@@ -285,4 +286,42 @@ describe('getAgentPrompt', () => {
 
     expect(result).toEqual({ content: promptContent });
   });
+
+  it('returns prompt from DB when agent.prompt is set (no file needed)', async () => {
+    const dbPromptContent = '# DB Prompt\nThis is persisted in the database';
+    const mockManager = {
+      get: vi.fn().mockResolvedValue(makeAgentInfo({ name: 'test-agent', prompt: dbPromptContent })),
+    };
+
+    // workspaceRoot has no PROMPT.md — but DB value takes precedence
+    const ctx = createTestContext({
+      agentManager: mockManager as any,
+      workspaceRoot: tmpDir,
+    });
+    const caller = createCaller(ctx);
+    const result = await caller.getAgentPrompt({ id: 'agent-1' });
+
+    expect(result).toEqual({ content: dbPromptContent });
+  });
+
+  it('falls back to PROMPT.md when agent.prompt is null in DB', async () => {
+    const agentName = 'test-agent';
+    const promptDir = path.join(tmpDir, '.cw', 'agent-logs', agentName);
+    await fs.mkdir(promptDir, { recursive: true });
+    const fileContent = '# File Prompt\nThis is from the file (legacy)';
+    await fs.writeFile(path.join(promptDir, 'PROMPT.md'), fileContent);
+
+    const mockManager = {
+      get: vi.fn().mockResolvedValue(makeAgentInfo({ name: agentName, prompt: null })),
+    };
+
+    const ctx = createTestContext({
+      agentManager: mockManager as any,
+      workspaceRoot: tmpDir,
+    });
+    const caller = createCaller(ctx);
+    const result = await caller.getAgentPrompt({ id: 'agent-1' });
+
+    expect(result).toEqual({ content: fileContent });
+  });
 });

apps/server/trpc/routers/agent.ts

@@ -342,6 +342,22 @@ export function agentProcedures(publicProcedure: ProcedureBuilder) {
       .query(async ({ ctx, input }) => {
         const agent = await resolveAgent(ctx, { id: input.id });
 
+        const MAX_BYTES = 1024 * 1024; // 1 MB
+
+        function truncateIfNeeded(text: string): string {
+          if (Buffer.byteLength(text, 'utf-8') > MAX_BYTES) {
+            const buf = Buffer.from(text, 'utf-8');
+            return buf.slice(0, MAX_BYTES).toString('utf-8') + '\n\n[truncated — prompt exceeds 1 MB]';
+          }
+          return text;
+        }
+
+        // Prefer DB-persisted prompt (durable even after log file cleanup)
+        if (agent.prompt !== null) {
+          return { content: truncateIfNeeded(agent.prompt) };
+        }
+
+        // Fall back to filesystem for agents spawned before DB persistence was added
         const promptPath = path.join(ctx.workspaceRoot!, '.cw', 'agent-logs', agent.name, 'PROMPT.md');
 
         let raw: string;
@@ -357,13 +373,7 @@ export function agentProcedures(publicProcedure: ProcedureBuilder) {
           });
         }
 
-        const MAX_BYTES = 1024 * 1024; // 1 MB
-        if (Buffer.byteLength(raw, 'utf-8') > MAX_BYTES) {
-          const buf = Buffer.from(raw, 'utf-8');
-          raw = buf.slice(0, MAX_BYTES).toString('utf-8') + '\n\n[truncated — prompt exceeds 1 MB]';
-        }
-
-        return { content: raw };
+        return { content: truncateIfNeeded(raw) };
       }),
   };
 }