docs: clarify plugin install vs disable vs uninstall lifecycle

[jiaxinaspenlin-dotcom]

Addresses #136

What

Adds docs/plugin-lifecycle.md and a short pointer to it from the README.

The repo had no documentation explaining how install, disable, and uninstall differ, which can make cached plugin artifacts easy to mistake for enabled-state evidence.

Contents

• A lifecycle table distinguishing install, disable, and uninstall.
• An explicit note that cached plugin artifacts are not authoritative evidence that a plugin is currently enabled.
• Guidance to use Cursor's supported plugin-management surfaces, including Customize / Cursor Settings → Plugins, as the source of truth for enabled state.
• A short troubleshooting section recommending a new agent session after changing plugin state.

Scope

Deliberately conservative:

• No undocumented internal state-storage details such as state.vscdb keys.
• No hardcoded cache marketplace subdirectory names.
• No undocumented CLI subcommands.
• No claims about specific disk-deletion behavior on disable or uninstall.
• Product UX issues raised in Docs: plugin install vs disable vs uninstall lifecycle and enabled-state ground truth #136 are not changed here.

This is filed as Addresses #136 rather than Closes #136 because some parts of the issue concern product behavior rather than repository documentation.

Validation

npm install --no-save ajv ajv-formats
node scripts/validate-plugins.mjs

<!-- CURSOR_SUMMARY -->
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> [!NOTE]
> **Low Risk**
> Documentation-only change with no code, security, or runtime impact.
> 
> **Overview**
> Adds documentation that **install**, **disable**, and **uninstall** are separate actions, and that files in the plugin cache are not proof a plugin is enabled.
> 
> A new `docs/plugin-lifecycle.md` explains each action, points people at Cursor’s plugin UI/CLI as the source of truth, and covers common issues (stale sessions, missing plugins, local plugin layout). The README now links to that page.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit 626a91785e86737d8a7d00be54e79872ab181242. Bugbot is set up for automated code reviews on this repo. Configure [here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->