Add durable agent task board

[mimeding]

Summary

Addresses #1714 by adding a durable local task-board foundation for future spawned and remote agent orchestration.

• adds task, event, run, and dependency models
• stores the board in an always-encrypted SQLCipher database
• records append-only task events
• supports lease-based atomic claims, stale lease reclaim, and crash-style recovery
• rejects dependency cycles
• documents the storage threat model and the non-goals for this foundation PR

This does not add agent teams, channel inboxes, remote execution, or UI workflows.

Validation

• git diff --check
• OSAURUS_DISABLE_KEYCHAIN_FOR_TESTS=1 OSAURUS_TEST_ROOT=/tmp/osaurus-test-task-board swift test --package-path Packages/OsaurusCore --filter 'AgentTaskBoardStoreTests|StorageDatabaseCatalogTests|StorageOptInEncryptionTests'

Review Notes

• Claude review found no blocking issues after building the package targets and running the relevant storage tests.
• Grok compact review found no visible blocking issue in the shown foundation diff.

Risk

The board is storage/API only. Future callers should open it off the main thread because first open may touch the storage key. The docs state that SQLCipher protects offline database theft, not a compromised local process.

[tpae]

This one has risk of diversion in the upcoming changes.

[mimeding]

That makes sense. I intended this as a storage/API foundation only: encrypted task, event, run, and dependency records with lease/reclaim semantics. It does not add agent-team protocol behavior, routing, UI, channel inboxes, or remote execution.

If this risks pulling the upcoming orchestration design in the wrong direction, I am fine deferring or closing it and revisiting after the agent/channel protocol direction is settled.