feat(state): add read-only battery clamshell status

[LMLiam]

Goal

Read the Battery Power disablesleep value safely and expose it through clamshellctl status.

Acceptance criteria

• Parse only the Battery Power section of pmset -g custom.
• Treat a missing battery disablesleep key as disabled.
• Reject missing sections and unexpected values.
• Keep the status path read-only.
• Print a concise enabled or disabled result.

Owned files

ClamshellState.swift, PowerSettingsParser.swift, ProcessRunner.swift, PowerSettingsClient.swift, StatusCommand.swift, and their tests.

Dependencies

Package foundation from Task 1.

Verification

swift test --filter PowerSettingsParserTests
swift test --filter PowerSettingsClientTests
swift test
swift run clamshellctl status

Plan: Phase 2, Tasks 3–4