docs(3.4): maintain living compat matrix as capabilities land

[espetro]

What

docs-site/reference/compat-matrix.md was seeded in Stage 0. This tracking issue ensures the matrix stays current as items 3.1–3.3 land.

Acceptance criteria

• Matrix updated after 3.1 lands: subagentStart/subagentStop rows reflect OpenCode actual status (guided-per-harness or unsupported)
• Matrix updated after 3.2 lands: tools[] and mcpServers rows match adapter behavior
• Matrix updated after 3.3 lands: any handler-type corrections reflected
• Matrix is the authoritative reference linked from PRD and AGENTS.md

Classification

• Type: documentation
• Effort: S
• Branch: feat/v0.3.0-authoring-wins
• Milestone: v0.3.0 (Stage 1 — item 3.4)