chore(cursor-rules): simplify and reorganize cursor rules for CE (#41691)

## Description

Reorganize and simplify `.cursor/rules/` for the CE codebase:

- **Added** focused domain rules: `agent-behavior.mdc`, `backend.mdc`,
`frontend.mdc`, `infra.mdc`
- **Simplified** existing rules (`index.mdc`,
`semantic-pr-validator.mdc`, `performance-optimizer.mdc`,
`test-generator.mdc`, `bug-fix-verifier.mdc`, `feature-verifier.mdc`,
`workflow-validator.mdc`) — reduced verbosity and removed redundant
content
- **Removed** obsolete/redundant rules: `semantic-pr.md`, `index.md`,
`pre-commit-checks.mdc`, `task-list.mdc`,
`feature-implementation-validator.mdc`

Net result: **+443 / -3,764 lines** — leaner, more actionable rules.

## Communication
Should the DevRel and Marketing teams inform users about this change?
- [ ] Yes
- [x] No


Made with [Cursor](https://cursor.com)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Documentation**
* Added comprehensive stack and infrastructure documentation for
backend, frontend, and infra.
  * Added agent behavior, task-management, and workflow guidance.
* Replaced previously automated validation rules with static reference
checklists for PRs, testing, bug fixes, feature implementation,
performance, and workflow reviews.
* Consolidated and restructured rule indexing into an
architecture-oriented overview.
* **Chores**
  * Removed legacy executable rule implementations and activation hooks.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

<!-- This is an auto-generated comment: Cypress test results  -->
> [!WARNING]
> Tests have not run on the HEAD
e6c3b705224ab4dbf10ff680380a1d7debac94a4 yet
> <hr>Fri, 03 Apr 2026 10:55:16 UTC
<!-- end of auto-generated comment: Cypress test results  -->