docs: Add libsecret installation guide for Linux and future automation architecture

[Copilot]

📥 Pull Request

✨ Description of new changes

MSAL token cache encryption on Linux requires libsecret and a Secret Service provider. Users encountering EncryptionFailed errors had no guidance on resolving this.

Changes

• docs/troubleshooting.md: Added Linux libsecret installation section with:

  • Package manager commands for Ubuntu/Debian, Fedora/RHEL, Arch, openSUSE, Alpine
  • Keyring daemon startup instructions
  • Verification commands and common issues table
  • Architecture documentation for future CLI automation (distro detection, dependency checking, install flow diagram)

• src/fabric_cli/errors/auth.py: Improved encrypted_cache_error() message to include Linux-specific guidance and documentation link

• docs/essentials/cache.md: Added platform requirements table (Windows/DPAPI, macOS/Keychain, Linux/libsecret)

Updated error message

An error occurred with the encrypted cache. On Linux, ensure libsecret is installed 
(e.g., 'sudo apt install libsecret-1-0 gnome-keyring' on Ubuntu/Debian). 
See https://aka.ms/fabric-cli for other distributions. 
Alternatively, enable plaintext fallback with 'config set encryption_fallback_enabled true'

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