fix: preserve account state across OAuth pool transitions

[BH4EHN]

OAuth account-pool transitions could leave stale managers writing snapshots, lose pending changes after persistence failures, or install a manager before validating the persisted account.

This change confirms accepted saves before retiring a manager, captures direct-save snapshots at invocation time, and retains failed intents for guarded recovery. OAuth transitions validate authoritative storage before installing the replacement manager. Eligibility-state save failures now allow requests to continue through another usable account while preserving account restrictions, cancellation, and transition checks.

Validation

• 2,089 unit tests passed.
• 10 local OpenCode 2 host tests passed with a mocked upstream.
• Build, project typecheck, Biome formatting, and lint passed.
• Independent full-branch review found no actionable issues.
• Docker-isolated tests were not run locally because Docker is unavailable.

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Summary by cubic

Fixes account-state loss across OAuth pool transitions: stale managers can no longer write snapshots after being retired, pending changes survive persistence failures, and new managers are only installed after the persisted account is validated.

• Confirms accepted saves before retiring a manager.
• Captures direct-save snapshots at invocation time and on dispose.
• Retains failed save intents for guarded recovery after pool fences, with retries scoped to lock-contention failures.
• Validates authoritative storage before installing the replacement manager.
• Eligibility-state save failures fall through to another usable account while preserving restrictions and transition checks.
• Routes OpenCode pool reads and writes through core's lock-held persistence so concurrent saves and ordered writes survive transitions.

^{Written for commit 98220b4. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 5 files

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[cubic-dev-ai]

All reported issues were addressed across 4 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

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[cubic-dev-ai]

All reported issues were addressed across 4 files (changes from recent commits).

<sub>Reply with feedback, questions, or to request a fix.

View guided diff | Re-trigger cubic</sub>