feat(session): add storage interface for pluggable session backends

[ryanjwong]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This adds a typed storage contract for session persistence, wraps SQLite behind that contract, and makes the contract async-compatible so non-SQLite providers can be added more easily.

Changes included:

• Added SessionStore + SessionStoreTx interfaces in packages/opencode/src/session/store.ts, with operation groups for sessions/messages/parts/todos/permissions.
• Added schema-derived insert/select types (SessionInput, SessionRow, etc.).
• Added Asyncable<T> return support across the store interface (use, transaction, effect, and tx methods).
• Added SqliteSessionStore adapter in packages/opencode/src/session/store.sqlite.ts that wraps Database.use, Database.transaction, and Database.effect.
• Routed core write paths through the adapter:
  • Session.updateMessage, Session.removeMessage, Session.removePart, Session.updatePart
  • Todo.update and Todo.get (now async to align with async-capable store)
• Exported store types and sqlite adapter from packages/opencode/src/session/index.ts.

This remains incremental: runtime behavior should stay equivalent while introducing a real, async-ready boundary for follow-up backends.

How did you verify your code works?

• Installed Bun (1.3.10) in this environment
• Ran bun install at repo root
• Ran bun run typecheck in packages/opencode
• Pushed branch successfully; pre-push hook ran bun turbo typecheck and passed

Screenshots / recordings

N/A (no UI changes)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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