Show agent providers in session rows

[adamhulme]

Summary

• show Codex, Claude, or Custom at the start of each local Agents row description
• show the same friendly provider name for agents coordinated from other VS Code windows
• include the provider in tooltips and accessibility labels while preserving status, MCP activity, and attention icons
• document the change and cover all three provider types plus a remote session in the extension-host integration suite

Why

Provider identity was only reliably visible in the hover tooltip when an agent used a custom or renamed label. This made mixed Codex, Claude, and custom-agent sessions harder to distinguish at a glance.

User impact

Agents rows now read like Claude · repository · working, so provider identity is always visible without sacrificing the existing live-status iconography.

Validation

• npm run lint
• npm run compile
• npm test — 220 passing
• npm run test:integration — 14 passing