fix: directory-aware root detection in CopilotRule.fromFile

[dyoshikawa]

Motivation

• Prevent misclassification of non-root Copilot instruction files that share the same filename as the root file by respecting the provided directory context when available.

Description

• Make CopilotRule.fromFile() detect root vs non-root using relativeDirPath + relativeFilePath when relativeDirPath is provided, and fall back to filename-only detection for backward compatibility when it's not.
• Resolve and use resolvedRelativeDirPath for reading non-root files so the directory context is preserved during file I/O and instance creation.
• Align CopilotRule.forDeletion() root detection to the same directory-aware logic.
• Add tests to ensure that a file named copilot-instructions.md in a non-root directory is treated as non-root, and that root is detected only when both directory and filename match.

Testing

• Ran pnpm vitest run src/features/rules/copilot-rule.test.ts and the test file passed (62/62 tests passed).
• Ran full checks with pnpm cicheck, which completed successfully (format/lint/typecheck passed and the full test suite passed: 5262 tests).

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Codex Task

[dyoshikawa]

@dyoshikawa Thank you!