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/test/integration/crash-race-condition.test.ts

@@ -32,6 +32,7 @@ interface TestAgent {
   initiativeId: string | null;
   userDismissedAt: Date | null;
   exitCode: number | null;
+  prompt: string | null;
 }
 
 describe('Crash marking race condition', () => {
@@ -72,7 +73,8 @@ describe('Crash marking race condition', () => {
       pendingQuestions: null,
       initiativeId: 'init-1',
       userDismissedAt: null,
-      exitCode: null
+      exitCode: null,
+      prompt: null,
     };
 
     // Mock repository that tracks all update calls