[WIP] Update docs for agent host

[ntrogh]

Summary

This PR restructures the AI agent documentation around the Agent Host, agent harnesses, and sessions model.

• Consolidates the separate Local, Copilot, Claude/Codex, and cloud-agent pages into a unified agent harness guide.
• Adds conceptual documentation for agent harnesses, sessions, and handoff, plus a practical handoff guide.
• Adds a streamlined agent quickstart for both the Chat view and Agents window.
• Updates the AI-generated code review guidance for Agent Host sessions. Edits are saved directly and reviewed through diffs, Source Control, worktrees, or pull requests; the older pending-edit workflow remains documented as a legacy fallback.
• Refreshes terminology and cross-links throughout the agent, customization, security, enterprise, and getting-started documentation.
• Updates navigation, redirects, and the sitemap for the new information architecture.
• Adds guidance for using Copilot beyond VS Code, including terminal, GitHub, mobile, and remote workflows.

Most of the deleted content was consolidated into the new agent harness documentation rather than removed.

Validation

• Reviewed navigation and redirects for the restructured pages.
• Checked terminology and cross-references across the affected documentation.

[ntrogh]

Remaining tasks:

• Deploy and validate in test environment
• Add and update screenshots
• Full read-through and edit pass
• Revisit the agents tutorial to remove the local harness
• Fix issue Agents toc
• Add getting started tutorial for agent-first experience with AW
• Add tutorial for working only with BYOK