docs: add INSTALL.md with VSCode, Cursor, JetBrains install flows

[Gradata]

What

Consolidated install instructions into a single INSTALL.md at the repo root.

Sections

• VSCode — Claude Code extension + plugin clone + setup.js
• Cursor — identical flow, plus activation-event notes
• JetBrains — SDK-only CLI workflow (no plugin hook wiring yet), manual capture/promote/export

Each section has install steps, a verify command, a post-install checklist, and expected output so users know it worked.

Verification

• Follows AGENTS.md style rules (markdown, no tables, POSIX paths, code fences with language)
• JetBrains section is honest about no plugin wiring — shows CLI workflow instead of aspirational docs

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on base/target branches other than the default branch.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 89193cde-6b9a-4407-ad1b-14fcadf2c97b

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/install-md-3-ides

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