docs: clarify managed-safe DMC capabilities

[chubes4]

Summary

• Clarifies that DMC support on managed hosts depends on the subsystem rather than the host label.
• Documents GitSync and GitHub API abilities as API-first surfaces that do not require shell or local git.
• Updates Environment documentation so DMC activation is distinguished from shell/workspace capability checks.

Verification

• php -l inc/Environment.php
• git diff --check -- README.md inc/Environment.php

Notes

• homeboy lint data-machine-code --path /Users/chubes/Developer/data-machine-code@docs-managed-safe-dmc still reports existing PHPCS alignment warnings in inc/Workspace/Workspace.php; this PR does not touch that file.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Drafted the documentation updates and verification summary; Chris identified the stale managed-host framing and remains responsible for review.