feat(FN-2115): add heartbeat multiplier and agent interval controls

- Add heartbeatIntervalMultiplier to shared settings schema/types with settings parity coverage
- Apply heartbeat multiplier in engine scheduling logic while preserving explicit per-agent interval behavior
- Add Settings modal and Agents view controls for heartbeat multiplier and per-agent interval overrides, including new styling
- Expand dashboard and engine test coverage for multiplier/heartbeat controls and document the new setting

docs/agents.md

@@ -111,6 +111,7 @@ The `runtimeConfig` field on agents supports the following options:
 | `budgetConfig` | `AgentBudgetConfig` | — | Token budget governance settings |
 
 Heartbeat values are validated and minimum-clamped.
+Project setting `heartbeatMultiplier` (default `1`) scales resolved heartbeat intervals globally; per-agent `heartbeatIntervalMs` remains the base interval before multiplier scaling.
 
 ## Agent Instructions (Dashboard)
 

docs/settings-reference.md

@@ -80,6 +80,7 @@ Defaults from `DEFAULT_PROJECT_SETTINGS`; key scope from `PROJECT_SETTINGS_KEYS`
 | `globalMaxConcurrent` | `number` | `4` | System-wide max concurrent agents across all projects. |
 | `maxWorktrees` | `number` | `4` | Max git worktrees. |
 | `pollIntervalMs` | `number` | `15000` | Scheduler poll interval (ms). |
+| `heartbeatMultiplier` | `number` | `1` | Global multiplier applied to all agent heartbeat intervals. |
 | `groupOverlappingFiles` | `boolean` | `true` | Serialize execution when file scopes overlap. |
 | `autoMerge` | `boolean` | `true` | Auto-finalize tasks from `in-review`. |
 | `mergeStrategy` | `"direct" \| "pull-request"` | `"direct"` | Completion mode (local direct merge vs PR-first). |