Fix specify-rules.mdc format for Cursor IDE

[jereimers]

The generated specify-rules.mdc file does not follow Cursor project rules formatting guidelines. Without the cursor-rules header, the rule type defaults to "manual", meaning that agent chats won't have the top-level rule included in their contexts unless the user manually includes this.

It's possible the integration with Cursor could get better with smarter use of their rule-types (imagine auto-including test-related rules when a *-test.js file is added to chat).

In any case, the fix is easy enough that I think it should be included in the next release. Just prepend the specify-rules.mdc with a header like this:

---
description: Project Development Guidelines
globs: ["**/*"]
alwaysApply: true
---

[fsilvaortiz]

I've submitted a fix for this in PR #1699.

Root cause: The update-agent-context.sh (and its PowerShell equivalent) generates Cursor's .mdc file from the same markdown template used by all agents, but Cursor requires YAML frontmatter with alwaysApply: true for rules to be auto-included. Without it, the rules file is silently ignored.

Fix: Both scripts now prepend the required frontmatter block when the target file is .mdc, handling three scenarios:

1. New file creation — frontmatter is prepended after template processing
2. Existing file without frontmatter — frontmatter is added during update
3. Existing file with frontmatter — no duplication (checked via head -1 / first-line comparison)

The generated frontmatter:

---
description: Project Development Guidelines
globs: ["**/*"]
alwaysApply: true
---

🤖 Generated with Claude Code