Fix chat fanout + unify system-injected agent prompts (#830)

* Fix chat everyone fanout

* Unify system-injected agent prompts

Chat mentions, schedule fires, and notifyOnFinish now share one shape:
prompt wrapped in <paseo-system>...</paseo-system>, no user-message
turn recorded, no auto-unarchive. Agents can recognize system-injected
context vs a real user turn.

- Move agent prompt orchestration out of mcp-shared.ts into a neutral
  agent-prompt.ts module (mcp is an interface, not the home).
- Add `unarchive` and `recordUserMessage` flags to sendPromptToAgent.
- Add formatSystemNotificationPrompt helper.
- Schedule fires now identify themselves: "Schedule \"<name>\" fired
  (id=..., run=...)." Plumb runId through runner so the paseo.schedule-run
  label matches the actual run record.
- Replace chat-mention validateChatMentionFanout + duplicate
  resolveChatMentionTargetAgentIds with single prepareChatMentionFanout
  that owns expansion + cap + poster snapshot.

packages/server/src/server/agent/mcp-server.ts

@@ -60,13 +60,11 @@ import {
   parseDurationString,
   resolveRequiredProviderModel,
   sanitizePermissionRequest,
-  sendPromptToAgent,
-  setupFinishNotification,
   serializeSnapshotWithMetadata,
-  startAgentRun,
   toScheduleSummary,
   waitForAgentWithTimeout,
 } from "./mcp-shared.js";
+import { sendPromptToAgent, setupFinishNotification, startAgentRun } from "./agent-prompt.js";
 import type { GitHubService } from "../../services/github-service.js";
 import type { WorkspaceGitService } from "../workspace-git-service.js";
 import type { CreatePaseoWorktreeInput } from "../paseo-worktree-service.js";