fix: drop root frontmatter when building CopilotRule

[dyoshikawa]

Motivation

• Ensure root Copilot files do not carry frontmatter in-memory to match the comment and serialized format and to avoid injecting globs/description during generate→import roundtrips.
• This aligns behavior with the intent documented in code and prevents idempotency issues described in [Bug]: Root CopilotRule receives frontmatter object despite "no frontmatter" comment #1545.

Description

• Stop passing the computed copilotFrontmatter for root rules in CopilotRule.fromRulesyncRule() and explicitly pass frontmatter: {} for root conversions.
• Keep non-root conversions unchanged and continue to populate copilotFrontmatter for non-root files.
• Update unit tests in src/features/rules/copilot-rule.test.ts to assert root conversions return an empty frontmatter and no longer preserve description, globs, or copilot.excludeAgent via the root conversion path.
• Minor test adjustments to reflect the new expected shapes for root rule roundtrips.

Testing

• Ran the focused test file with pnpm vitest run src/features/rules/copilot-rule.test.ts and it passed (60/60 tests).
• Ran full CI checks with pnpm cicheck, which includes formatting, linting, typecheck and the full test suite, and all automated checks passed (5260 tests passed, no failures).

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Codex Task