feat(agents): surface heartbeat activity

Keep idle babysitting agents visible through a dedicated heartbeat track while projecting active heartbeat schedules into workspace activity. Agent lifecycle stays unchanged so heartbeat targets do not gain interrupt semantics.

docs/agent-lifecycle.md

@@ -111,7 +111,7 @@ Running provider-native subagents contribute `running` to the workspace owned by
 
 ## The subagents track
 
-The collapsible track above the composer in an agent's pane (`packages/app/src/subagents/track.tsx`) combines two kinds of children:
+One collapsible section of the composer track lane in an agent's pane (`packages/app/src/subagents/track.tsx`), stacked with any other track the agent has. It combines two kinds of children:
 
 - **Paseo subagents** are full managed agents. Their membership rule (`packages/app/src/subagents/select.ts`) is:
 

docs/glossary.md

@@ -30,7 +30,7 @@ Authoritative terminology. UI label wins. Don't invent synonyms; use what's here
 - **Tab** — UI surface representing one session inside a workspace. Not a conceptual unit; use **Agent session** when talking about the model. Code: `WorkspaceTabDescriptor` (`packages/app/src/screens/workspace/workspace-tabs-types.ts`).
 - **Terminal** — Workspace-scoped PTY shell streamed over the binary mux channel. UI: "Terminal". Code: `TerminalStreamFrame` (`packages/protocol/src/terminal-stream-protocol.ts`).
 - **Schedule** — Cron-style trigger that creates new agents. UI: CLI/MCP (`paseo schedule`, `create_schedule`). Don't confuse with: Heartbeat (cron prompt back into the same agent) or Loop (iterative re-execution of one agent).
-- **Heartbeat** — Ephemeral cron prompt sent back into the same agent/conversation. Agent surfaces expose create, update cron, and delete only. Use for reminders and babysitting where status should return inline.
+- **Heartbeat** — Ephemeral cron prompt sent back into the same agent/conversation. Agent surfaces expose create, update cron, and delete only. Use for reminders and babysitting where status should return inline. The agent's **Heartbeats track** lists the ones still on a cadence for that agent on that host; opening a row goes to it on Schedules (`/schedules?serverId=…&scheduleId=…`, keyed by both because schedule ids are host-local). Paused, completed, and expired heartbeats stay on Schedules and leave the track.
 - **Mode** — Provider-specific operational mode (plan, default, full-access, …). UI: icon-only. Code: `modeId` in `AgentSessionConfig` (`packages/protocol/src/messages.ts:257`).
 - **Attachment** — External or local context bound to an agent prompt: forge issue/change request, review context, uploaded file, text, or image. UI: "Attach issue or PR/MR". Code: `AgentAttachment` (`packages/protocol/src/messages.ts:782`).
 - **Composer** — The whole prompt surface for sending work to an agent. Code: `Composer` (`packages/app/src/composer/index.tsx`). Don't call this "message input" except for the text-entry subcomponent.
@@ -38,7 +38,7 @@ Authoritative terminology. UI label wins. Don't invent synonyms; use what's here
 - **Composer toolbar** — The bottom control row inside the composer input. Contains agent controls, attachment button, voice controls, and stop/send controls. Code: `leftContent`, `beforeVoiceContent`, and `rightContent` slots in `MessageInput` (`packages/app/src/composer/input/input.tsx`). Forbidden: "Status bar".
 - **Agent controls** — Provider, model, mode, thinking, and provider-feature controls for an agent or draft agent. Code: `AgentControls` / `DraftAgentControls` (`packages/app/src/composer/agent-controls/index.tsx`). Forbidden: "Agent status bar".
 - **Composer footer** — Optional area rendered below the composer input but still inside the keyboard-shifted composer layout. Code: `Composer.footer` (`packages/app/src/composer/index.tsx`).
-- **Composer track** — A contextual lane above the composer input. Specific tracks use the `<thing> track` form: **Queue track**, **Subagents track**. Code: queue track inside `Composer` (`packages/app/src/composer/index.tsx`), `SubagentsTrack` (`packages/app/src/subagents/track.tsx`).
+- **Composer track** — A contextual lane above the composer input. Specific tracks use the `<thing> track` form: **Queue track**, **Heartbeats track**, **Subagents track**. Tracks above the input are sections of one card so several can stack without either one owning the seam with the composer. Code: `ComposerTracks` / `ComposerTrackSection` (`packages/app/src/composer/tracks.tsx`), queue track inside `Composer` (`packages/app/src/composer/index.tsx`), `HeartbeatsTrack` (`packages/app/src/heartbeats/track.tsx`), `SubagentsTrack` (`packages/app/src/subagents/track.tsx`).
 - **Subagent** — User-facing term for an agent session related to a parent agent session. Use **subagents** in UI copy and docs. Internal daemon/provider plumbing may say "child agent" or `child_session`, especially for provider-managed imports; do not surface "child agent" as a product term.
 - **Attachment tray** — The selected-attachments row inside the composer input, above the text input. Code: `renderAttachmentTray` (`packages/app/src/composer/index.tsx`). Forbidden: "Attachment bar".
 - **Fork** — Starting a NEW agent session seeded with a copy of another session's curated chat history. The history rides along as a `chat_history` composer attachment on a draft; the fork does **not** resume or branch the provider-side session, and the source session keeps running untouched. Two targets, one surface, one RPC (`agent.fork_context`, gated on the `agentForkContext` host feature). UI: the fork menu lives in the assistant turn footer and offers "Fork in a new tab" / "Fork in a new workspace". On a **completed** turn it sits beside the copy button and pins a boundary to that turn, so the fork stops where the turn ended. On the **in-flight** turn it sits beside the progress loader and elapsed timer, and omits the boundary entirely — `selectForkContextRows` projects the whole timeline when no boundary is given, so the fork captures everything **up to now**, including the response still streaming. That omission is what makes mid-run forking work; pinning a boundary there would silently drop the live response. Forbidden on provider-subagent (read-only) panes. Code: `useForkAgent` (`packages/app/src/hooks/use-fork-agent.ts`), `selectForkContextRows` (`packages/server/src/server/agent/activity-curator.ts`). Don't confuse with: git fork (unrelated; Paseo has no such concept), **Worktree** (filesystem isolation, not history copying).