docs: rewrite device agent documentation (#2162)

* docs: rewrite device agent documentation for new agent

Replaces outdated FleetDM/MDM-based docs with comprehensive guide
covering the new Comp AI Device Agent: installation (macOS, Windows,
Linux), sign-in flow, status window, remediation options, tray menu,
troubleshooting, and manual evidence collection. Added Linux manual
evidence section.

Image placeholders:
- device-agent-sign-in.png (sign-in window screenshot)
- device-agent-status.png (status window with checks screenshot)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add device agent screenshots

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: fix swapped device agent screenshots

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add uninstall guide for old Fleet agent

Adds section with platform-specific instructions for removing the
old FleetDM-based agent (MDM profile, orbit, osqueryd) on macOS,
Windows, and Linux.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: rewrite fleet uninstall section based on actual codebase

Uses real paths from git history (scripts/mac.ts, scripts/windows.ts,
fleet-label.ts, download route): /Users/Shared/.fleet for macOS,
C:\ProgramData\CompAI\Fleet and HKLM\SOFTWARE\CompAI\Device for
Windows. Removed Linux section (old Fleet agent was macOS/Windows
only). Added context about run_me_first scripts and marker files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: simplify old agent uninstall section with FleetDM guide link

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: rename section to Migrating from FleetDM, remove admin note

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: tofikwest