Audit remaining snapshot/restore failure paths for app.log coverage

[Kosinkadink]

Follow-up to #1248 and #1250. #1248 improved failure logging for snapshot restore (shared restore-failure helper, dedup of failure logging, containment of repair-pass rejections as repair errors), but the fix was scoped to the restore path.

Remaining work: audit the other snapshot and env-management codepaths for #1250-style gaps where errors surface in the UI (or are swallowed) without being recorded in the install''s app.log:

• snapshot capture/export/import/staging paths
• torch stack switching (change-pytorch) failure paths
• create-install-from-snapshot fallback paths (including the pruned-R2-bundle fallback added in Add trusted PyTorch stack switching and reliable v2 snapshot torch restore #1248)
• migration/adoption scanner errors

Each user-visible failure should leave enough evidence in app.log to diagnose without asking the user to reproduce.

[coderabbitai]

🔗 Related PRs

#1206 - fix(snapshots): keep newest snapshot accurate after failed/rolled-back restore [open]
#1235 - Telemetry: standardize error events (matched error, class, normalized signature; stderr tail for boot_failed) [merged]
#1247 - Define e2e/lifecycle test taxonomy, inventory specs, and run lifecycle tests nightly [open]
#1248 - Add trusted PyTorch stack switching and reliable v2 snapshot torch restore [open]

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