Align provider installation guidance

[bucurdavid]

Summary

• Rework README installation instructions into provider-specific sections for Claude Code, Codex, Cursor, OpenCode, Gemini CLI, and skill-only installs.
• Align plugin, marketplace, package, and Gemini extension descriptions with the shared skill-optimization positioning.
• Refresh contributor and agent context docs to reflect the current workbench layout and install-doc invariants.

Test Plan

• npx tsx tests/smoke-skill-distribution.ts
• npm run typecheck
• npx tsx src/cli.ts --help
• npm pack --dry-run --json
• git diff --check