Build complete agentctl documentation system

[Ompragash]

Summary

• establish canonical installation, workflow, operations, reference, use-case, architecture, security, and contributor documentation
• add verified workflow examples and provider variants
• add documentation verification to the Rust xtask and CI gate

Impact

The agentctl repository becomes the source of truth for the public documentation website and provides executable, versioned examples for users and contributors.

Validation

• cargo xtask verify
• cargo xtask docs-verify
• cargo xtask secret-scan
• cargo xtask acceptance-container
• clean-directory tutorial and deterministic fake-provider checks