aimgr repo sync exits 0 when configured sources fail partially

[hk9890]

Bug\n\naimgr repo sync now reports per-source failures in its output, but it still exits successfully unless all configured sources fail. This means automation cannot detect a partial sync failure even when one or more configured sources were not refreshed.\n\n## Steps to Reproduce\n\n1. Configure two sync sources:\n - one valid source\n - one invalid or unreachable source\n2. Run:\n\nbash\naimgr repo sync --format=json\n\n\n## Actual Behavior\n\nThe command exits with status 0 even though the JSON output reports failed sources, for example:\n\njson\n{\n "summary": {\n "sources_total": 2,\n "sources_synced": 1,\n "sources_failed": 1\n }\n}\n\n\nPer-source error details are present in the output, but the process exit status still signals success.\n\n## Expected Behavior\n\n- Exit non-zero when any configured source fails to sync\n- Preserve per-source error details in table/json/yaml output\n- Keep successful sources applied, but make the partial failure visible to scripts and CI\n\n## Impact\n\nThis breaks automation and CI workflows that rely on the process exit status to detect whether a sync completed fully. A partially refreshed repository can currently be treated as success.\n\n## Notes\n\nThis issue is no longer primarily about silent failure: current code already includes per-source failure details. The remaining bug is the success exit code on partial failure.

[hk9890]

Reproduced on current main. I created a temp repo with one valid local source and one invalid path, then ran ./aimgr repo sync --format=json. The command exited 0 even though the JSON output included a failed source and "sources_failed": 1.\n\nI think this is a real bug worth fixing because the repository was only partially refreshed, but callers still get a success exit code. Good fix plan: preserve the existing per-source output, but return a non-zero process status when any configured source fails. The safest contract is likely: 0 = full success, 1 = sync completed with failed sources / incomplete result, 2 = operational failure that prevented the command from completing at all. Tracked locally in beads as ai-config-manager-buch.