fix(stats): skip corrupt decided/*.yaml so vouch stats cannot crash

[kai392]

Summary

Root cause: stats._list_decided() — used by review_summary / collect_stats (vouch stats, kb.stats) — parsed every decided/*.yaml strictly. One corrupt or hand-edited decided file raised an unhandled exception and aborted the whole stats surface.

Meanwhile KBStore.list_proposals() already uses _load_or_skip, and list_pages() gained the same resilience in #360. _list_decided was the leftover outlier.

Fix: Use _load_or_skip when loading decided proposals. One bad file logs a warning and is skipped; valid decisions still count.

Impact: vouch stats / kb.stats keep working when a single decided proposal YAML is unreadable.

Reproduction

# After one successful approve, plant garbage under decided/:
(store.kb_dir / "decided" / "corrupt.yaml").write_text("not: [valid\n")
stats.review_summary(store, since_days=None)
# before: yaml.ParserError
# after:  approved count still includes the good decision

Risk / tradeoffs

• Corrupt decided files are omitted from review totals (same behavior as corrupt pending proposals via list_proposals). Audit-based audit_totals in the same response are unaffected.

Test plan

• test_review_summary_skips_corrupt_decided_file (new)
• tests/test_stats.py pass

[plind-junior]

@kai392 Next time, raise the issue on GH for such kinda valid discoveries. It's worth to be rewarded as a discovery