Clarify install, deploy, agent self-check, and optional skills

[lisihao]

Summary

• Add a dedicated agent install/deploy/self-check path to the GitHub homepage.
• Clarify that ./install.sh installs L1 Solar Core and syncs L2 Solar Harness when harness/ is present.
• Add .env.template for optional API-backed features; install does not require API keys.
• Rewrite INSTALL-AGENT.md as an authoritative agent-facing install/deploy/self-check protocol.
• Rewrite SKILLS-INSTALL.md to distinguish bundled Solar skills from optional third-party skill packs.
• Update install.sh comments and success output so it no longer claims fixed/huge third-party skill counts.

Notes

The plugin framework is installed as Harness code. Public docs now avoid implying that enabled third-party plugins are bundled by default; plugin manifests must exist and pass validation before a plugin should be treated as usable.

Follow-up

• USER-GUIDE.md still contains older L1/L2 wording and should be refreshed in a separate pass.
• Add a release/privacy scan gate for local machine paths, host fingerprints, and runtime excerpts.