feat: Add tiered coordination mode with multi-model consensus

Implement comprehensive fleet coordination features:

Core Types (aof-core):
- Add Tiered coordination mode for tier-based parallel execution
- Add HumanReview consensus algorithm for human-in-the-loop decisions
- Add agent weights and min_confidence to ConsensusConfig
- Add TieredConfig with per-tier consensus and final aggregation
- Add tier and weight fields to FleetAgent

Consensus Engine (aof-runtime):
- Implement all 5 consensus algorithms: Majority, Unanimous, Weighted,
  FirstWins, HumanReview
- Create ConsensusEngine with proper weighted vote calculation
- Add comprehensive unit tests for all algorithms

Tiered Execution:
- Implement execute_tiered() for tier-based parallel execution
- Agents grouped by tier, execute in parallel within each tier
- Results passed to next tier with configurable consensus
- Support manager_synthesis final aggregation

Example Agents:
- Add 4 observability agents: loki, prometheus, k8s, git collectors
- Add 3 reasoning agents: claude, gemini, gpt4 analyzers
- Add rca-coordinator manager agent
- Create multi-model-rca-fleet.yaml example

Documentation:
- Update docs/concepts/fleets.md with Tiered mode and all algorithms
- Add docs/tutorials/multi-model-rca.md comprehensive tutorial
- Add docs/reference/fleet-spec.md complete YAML specification
- Add examples/agents/README.md agent catalog

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: gouravjshah

crates/aof-core/src/fleet.rs

@@ -38,10 +38,10 @@ pub use workflow::{
 };
 pub use fleet::{
     AgentFleet, AgentInstanceState, AgentInstanceStatus, AgentRole, CoordinationConfig,
-    CoordinationMode, ConsensusConfig, ConsensusAlgorithm, FleetAgent, FleetAgentSpec,
-    FleetMetadata, FleetMetrics, FleetSpec, FleetState, FleetStatus, FleetTask, FleetTaskStatus,
-    SharedResources, SharedMemoryConfig, SharedMemoryType, CommunicationConfig, MessagePattern,
-    TaskDistribution, ScalingConfig,
+    CoordinationMode, ConsensusConfig, ConsensusAlgorithm, FinalAggregation, FleetAgent,
+    FleetAgentSpec, FleetMetadata, FleetMetrics, FleetSpec, FleetState, FleetStatus, FleetTask,
+    FleetTaskStatus, SharedResources, SharedMemoryConfig, SharedMemoryType, CommunicationConfig,
+    MessagePattern, TaskDistribution, ScalingConfig, TieredConfig,
 };
 
 /// Version information