Replace agent directory env var with kagent API

[jpower432]

Problem / Motivation

_load_agent_cards() in workbench/app.py reads agent metadata from the AGENT_DIRECTORY env var (a JSON blob). This is a static snapshot that must be manually kept in sync with what's actually deployed. kagent already maintains a live agent registry from Kubernetes CRDs and exposes it via API.

Proposed Solution

Replace the env var lookup with a call to kagent's agent listing API. The workbench becomes a consumer of kagent's registry rather than maintaining its own.

Changes:

• workbench/app.py:_load_agent_cards() — call kagent API instead of parsing env var
• workbench/app.py:agent_directory() — pass through kagent response, optionally enrich with studio-specific metadata
• agents/assistant/delegate.py:_resolve_agent_url() — resolve via kagent instead of workbench directory

Acceptance Criteria

• Workbench retrieves agent list from kagent at runtime
• AGENT_DIRECTORY env var removed or made fallback-only
• Delegation client resolves agents through kagent
• Existing tests updated
• Works when kagent is unreachable (graceful degradation)

Related

• Design: Evaluate dropping workbench A2A proxy in favor of kagent routing #7 — Design: Evaluate dropping A2A proxy (overlapping concern)
• Epic: Agent Infrastructure #2 — Epic: Agent Infrastructure

[jpower432]

Update: Evaluate MCP resource subscriptions first

kagent v0.9.6 shipped feat(mcp): expose agents as MCP resource with change notifications (kagent#1856). This means agents are now available as MCP resources with push-based change notifications.

Proposed approach change: Before building a REST client for /api/agents, spike on whether MCP resource subscriptions can replace the static directory. Benefits:

• Push-based instead of polling — no custom caching/refresh logic
• No HTTP client to maintain
• Aligns with studio's existing MCP tool consumption pattern

Spike tasks:

1. Review the MCP resource schema kagent exposes — does it include the fields we need (role, delegatable, tools, examples)?
2. Test resource subscription lifecycle in a local kagent deployment
3. Decide: MCP-only, REST-only, or MCP with REST fallback

If MCP resources don't cover our needs, fall back to the REST client approach described in the original issue.