Show conflicted file names in pull-failure guidance

[ori0o0p]

Summary

• improve Git panel pull conflict guidance by including conflicted file names when available
• reuse existing conflict detection helper (git diff --diff-filter=U) to provide actionable context
• keep fallback guidance when file enumeration fails

Why

Current guidance tells users to resolve conflicts but does not show which files are conflicted, forcing extra manual commands.

Scope

• Sources/Zero/Services/GitPanelService.swift
• Tests/ZeroTests/GitPanelServiceTests.swift

Acceptance Criteria

• pull conflict failures include conflicted file paths in the message when detected
• message remains concise and readable for multiple files
• fallback generic conflict guidance remains for empty/unknown conflict file results