Guard Claudian command checks during startup

[YishenTu]

Summary

• Adds a guarded Claudian view resolver before tab command callbacks access getTabManager().
• Keeps tab commands unavailable while Obsidian has restored a Claudian leaf whose view has not initialized yet, instead of throwing during command palette enumeration.
• Adds integration coverage for new-tab, new-session, and close-current-tab in that startup state.

Fixes #604

Tests

• npm run test -- tests/integration/main.test.ts --runInBand
• npm run typecheck