agentbridge: update README with all 5 install methods + PRD/plan sync

[espetro]

Context

The v0.2.0 pivot needs to be reflected in the public-facing docs.

Tasks

• Update README with 5 install methods side-by-side: native binary, npx, brew, curl, mise
• Update docs/prd.md roadmap section (already done in this commit, but verify)
• Update docs/plan.md strategic plan (already done in this commit, but verify)
• Add a "Quick start" section that shows agentbridge add <github-url> as the headline
• Add a "Distribution" page explaining the universal store + symlink model
• Mention Skills.sh compatibility in the README

Source

• README.md (modify)
• docs/prd.md (already updated)
• docs/plan.md (already updated)
• Plan: .agents/plans/2026-06-14-v0.2.0-distribution-pivot.md