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>

docs/database.md

@@ -70,6 +70,7 @@ All adapters use nanoid() for IDs, auto-manage timestamps, and use Drizzle's `.r
 | mode | text enum | 'execute' \| 'discuss' \| 'plan' \| 'detail' \| 'refine' |
 | pid | integer nullable | OS process ID |
 | exitCode | integer nullable | |
+| prompt | text nullable | Full assembled prompt passed to agent at spawn; persisted for durability after log cleanup |
 | outputFilePath | text nullable | |
 | result | text nullable | JSON |
 | pendingQuestions | text nullable | JSON |