Remove legacy fallback from Git diff rendering

[jsgrrchg]

Summary

• keep text, binary, empty, and invalid Git diffs inside the Pierre CodeView path
• represent unsupported textual diffs as labeled file placeholders without degrading the whole stack
• remove the legacy Git diff components and use a safe error state for unexpected renderer failures
• update Git and worktree diff coverage for mixed stacks and placeholder behavior

Testing

• pnpm run typecheck
• targeted ESLint for modified files
• pnpm test (297 files, 2701 passed, 1 skipped)