Add Continue.dev row to primitives matrix

[cobusgreyling]

Goal

Document Continue.dev in the primitives matrix for contributors using VS Code plus Continue.

Files

• docs/primitives-matrix.md

Acceptance criteria

• Add Continue.dev column or dedicated appendix
• Cover scheduling, skills path, STATE.md convention, maker/checker split
• Honest note if a primitive is not first-class in Continue yet

Estimated time: ~45 minutes

[cobusgreyling]

Community announcement with help-wanted table and merge highlights: #130 — comment "I'll take this" here to get assigned.