[Feature Request] Cross-instance agent handoff — Agent A hands off investigation to Agent B

## Problem

Azure SRE Agent supports **custom agent (sub-agent) handoffs within the same parent agent**, but there is no native mechanism for **cross-instance handoff** — where Agent A (e.g., an ops orchestrator) hands off an investigation to Agent B (e.g., a product-specific agent) running as a separate Azure resource.

## Current Behavior

- Custom agents within the **same parent** can hand off via handoff rules — this works well for domain specialization within a single agent
- Cross-instance handoff (Agent A → Agent B, where A and B are separate `Microsoft.App/agents` resources) is **not supported**
- Workaround: Agent A creates a work item with full context (evidence, KQL results, thread link), and the engineer manually opens a new thread on Agent B referencing the work item

## Proposed Solution

1. **Native cross-instance handoff API** — Agent A invokes Agent B via a platform-managed handoff, passing context (thread summary, evidence, resource IDs, severity)
2. **Shared investigation context** — the receiving agent gets a structured context payload, not just a raw text dump
3. **Bidirectional handoff** — Agent B can hand back to Agent A with results (e.g., product agent completes RCA, hands result back to ops agent for mitigation)
4. **Audit trail** — both agents' threads link to each other for traceability

## Use Case

In a **multi-agent topology**, a central operations agent detects issues across all products, while specialized product agents have deep domain knowledge and connected product repos. When the ops agent detects an exception burst in a specific product, it should hand off to the product-specific agent which has the relevant source code connected, product-specific skills, and architecture knowledge. Today this requires manual human intervention to bridge the two agents.

## Impact

- **High** — critical for multi-agent topologies where specialization is distributed across agent instances
- Enables true **agentic orchestration** patterns (orchestrator + specialists)
- Reduces MTTR by eliminating the manual bridge between agents

## Environment

- Azure SRE Agent (GA)

No repository files, tests, or implementation entry points are named. Start by reviewing the existing same-parent handoff behavior and the proposed cross-instance requirements; done would include a supported handoff with structured context, return routing, and linked audit trails.

Written by the indexing model from the issue text.