Surface user-friendly git clone error messages

[fedacking]

Summary

• Capture stderr in run_cmd_streaming: Collects stderr lines into a buffer alongside the existing broadcast, and includes them in the error message on failure.
• Map raw git errors to friendly messages: New map_git_clone_error() function translates opaque stderr (e.g., exit status: 128) into actionable messages like "Repository not found" or "Authentication failed".
• Wrap both clone call sites (initial task + reopen) to produce structured {friendly}\n---STDERR---\n{raw} error messages.
• Default failed tasks to Info tab instead of Diff tab, so users see the error immediately.
• Enhanced error display: Friendly summary with collapsible raw stderr details and an inline "Retry Task" button.
• 8 unit tests covering every error pattern + fallback.

Test plan

• Trigger a clone failure (e.g., invalid token, nonexistent repo) and verify the friendly error message appears in the Info tab
• Verify raw stderr is available in the collapsible details section
• Verify "Retry Task" button is visible and calls reopen
• View an older failed task to confirm backward compatibility (error displays as-is, no collapsible section)
• Run cargo test tasks::tests — all 8 tests pass

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