docs: fill DEV_GUIDE.md {FILL IN} onboarding stubs

[w7-mgfcode]

GitHub-presence audit (D2): docs/_base/DEV_GUIDE.md is the human-onboarding doc but ships 8 unfilled {FILL IN} placeholder sections plus the stub-marker line. Fill all sections with content grounded in current repo facts and remove the stub marker.

[w7-mgfcode]

Resolved — merged into dev (PR #141 / #143). Auto-close did not fire because the PRs targeted dev, not the default branch.