docs(FN-1158): add comprehensive project documentation suite

- Add docs index with structured navigation and links across the new documentation set
- Document onboarding and technical foundations with getting-started and architecture guides
- Add detailed CLI and settings references plus feature guides for dashboard, tasks, workflow steps, missions, multi-project, and agents
- Add a contributing guide and polish cross-references between docs pages for discoverability
- Include dashboard screenshots for key views to provide visual context in the guides

docs/agents.md

@@ -0,0 +1,92 @@
+# Agents
+
+[← Docs index](./README.md)
+
+Fusion uses multiple agent roles for triage, execution, review, and merge workflows.
+
+## Agents View (Dashboard)
+
+The agents surface provides:
+
+- Agent list and status
+- Detail/config panels
+- Runtime metrics
+- Run history
+- Task assignment context
+
+![Agents view](./screenshots/agents-view.png)
+
+## Built-In Agent Prompt Templates
+
+Fusion includes built-in templates for role prompts:
+
+- `default-executor`
+- `default-triage`
+- `default-reviewer`
+- `default-merger`
+- `senior-engineer`
+- `strict-reviewer`
+- `concise-triage`
+
+These can be assigned per role using `agentPrompts.roleAssignments`.
+
+## Per-Agent Configuration
+
+Agents can be configured with:
+
+- Custom instructions
+- Heartbeat interval/timeout limits
+- Max concurrent heartbeat runs
+
+Heartbeat values are validated and minimum-clamped.
+
+## Configurable Agent Prompts (`agentPrompts`)
+
+`agentPrompts` project setting supports:
+
+- `templates[]`: custom prompt templates by role
+- `roleAssignments`: map role → template ID
+
+When no assignment is configured, Fusion falls back to built-in defaults.
+
+## Inter-Agent Messaging
+
+Messaging is available in dashboard mailbox UI and CLI.
+
+```bash
+fn message inbox
+fn message outbox
+fn message send AGENT-001 "Please prioritize FN-420"
+fn message read MSG-123
+fn message delete MSG-123
+fn agent mailbox AGENT-001
+```
+
+## Agent Spawning
+
+Executor sessions can spawn child agents through `spawn_agent`.
+
+Behavior:
+
+- Child agents run in separate worktrees
+- Parent/child relationship is tracked
+- Limits enforced:
+  - `maxSpawnedAgentsPerParent` (default 5)
+  - `maxSpawnedAgentsGlobal` (default 20)
+- Child sessions terminate when parent task ends
+
+## Heartbeat Monitoring and Trigger Scheduling
+
+Fusion’s `HeartbeatTriggerScheduler` supports three trigger types:
+
+- `timer` — periodic wake based on heartbeat interval
+- `assignment` — wake when task is assigned to agent
+- `on_demand` — manual run trigger (`POST /api/agents/:id/runs`)
+
+All triggers respect per-agent `maxConcurrentRuns` and produce structured wake context metadata.
+
+## Related Docs
+
+- [Workflow Steps](./workflow-steps.md)
+- [Settings Reference](./settings-reference.md)
+- [Architecture](./architecture.md)