Fix inaccurate frontmatter null-safety description in AGENTS.md

[Copilot]

The frontmatter null-safety description in AGENTS.md incorrectly stated that moveCard() "throws on undefined frontmatter", which misrepresents the actual code behavior.

Changes

• Updated line 139 to clarify that moveCard() doesn't check for null/undefined before calling IsObject(), which causes the error
• Specified the fix more precisely: initialize empty object when frontmatter is null/undefined

Before:

- Frontmatter null-safety: moveCard() throws on undefined frontmatter (should initialize empty object)

After:

- Frontmatter null-safety: moveCard() doesn't handle null/undefined frontmatter before IsObject() check (should initialize empty object when null/undefined)

The updated description matches the actual implementation at KanbanStateController.ts:172 where IsObject(frontmatter) is called without prior null/undefined guards.

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