docs(3.6): ecosystem page + 'rewriting for tier-1 parity' guide

[espetro]

What

Two new docs-site pages completing Stage 1:

Ecosystem page

Lists community rewrites installable via agentplugins add owner/repo:

• agentplugins-caveman
• agentplugins-ponytail
• (placeholders for goal, btw, flow — v0.4.0)

Porting guide

"Rewriting an existing plugin for Tier-1 parity" — encodes the decision tree:

1. Does universal codegen cover it across all Tier-1? → use it
2. Can all Tier-1 support it via the escape hatch? → guided per-harness pattern
3. TUI-grade fidelity only? → acceptable degradation; note in compat matrix
4. None of the above? → open a primitive proposal (v0.4.0+ scope)

Complements Epic #24's migrate SKILL.md guide.

Acceptance criteria

• docs-site/guide/ecosystem.md (or similar) listing the two rewrites with install commands
• docs-site/guide/porting.md with the decision tree and links to examples
• Both linked from VitePress sidebar
• Goes out with the v0.3.0 tag (blocked on 3.5 being green)

Classification

• Type: documentation
• Effort: S
• Branch: feat/v0.3.0-authoring-wins
• Milestone: v0.3.0 (Stage 1 — item 3.6)