docs: clarify Webcmd agent onboarding

[ankitranjan7]

Summary

• Moves first-use guidance earlier in the README and docs quickstart.
• Separates the universal CLI install from the agent-harness support install.
• Adds a harness table for Codex, Claude Code, Cursor, Gemini CLI, OpenCode, Hermes Agent, and generic skills-compatible agents.
• Clarifies that Codex and Claude Code users install native Webcmd plugins instead of also running npx skills add.

Reasoning

Webcmd has two different installation concerns: the executable CLI and the agent instructions that teach a harness how to use that CLI. Comparable projects document those as separate steps: Agent Browser uses a CLI plus one discovery skill, Playwright CLI documents CLI install plus skill install, and Codex/Claude Code both provide native plugin marketplace paths. This PR makes that mental model explicit for new users before they reach lower-level adapter details.

Industry practice followed

• Keep the universal executable install first.
• Put agent-specific setup in a table so users do not read irrelevant harness instructions.
• Treat Codex and Claude Code plugin installs and npx skills add as alternatives, not cumulative setup.
• Keep detailed workflow skills behind runtime discovery so the README remains short.

Follow-up dependency

This is intentionally a draft because the --skill webcmd and webcmd skills get ... paths are completed by the single-discovery-skill PR. The Codex plugin and Claude Code plugin PRs then provide native marketplace install paths for those two agents.

Testing

• Docs-only change; no runtime tests run.