refactor: standardize conversation engine protocol

[Devin-AXIS]

What changed

• Added a small conversation-engine contract and adapter registry at the Work/UI boundary.
• Kept OpenCode as the only default engine and moved all OpenCode event, snapshot, permission, question, prompt, and tool-part mapping behind its adapter.
• Standardized rendered conversation data on AI SDK UIMessage / UIMessageChunk plus typed control events for session state, errors, compaction, todos, permissions, questions, message updates, part updates, and streaming chunks.
• Removed the replaced OpenCode-only session and chat adapter paths instead of retaining compatibility copies.
• Preserved the current user experience and OpenCode runtime behavior while making a future DSH adapter independently pluggable.

Why

The UI and session synchronization layer was directly coupled to OpenCode types and event shapes. This change creates one narrow engine boundary without adding a second UI data flow or changing the current default engine.

Impact

• No intended user-visible behavior change.
• OpenCode remains the default and only registered conversation engine.
• Future engines can implement the same Work-layer contract without changing session UI components.
• PR contains one commit and only app conversation/session files and their tests.

Validation

• pnpm --filter @ipollowork/app typecheck — passed.
• Focused conversation/session suite — 69 passed, 0 failed.
• pnpm --filter @ipollowork/app build — passed.
• Maintainability audit and git diff --check — passed with no errors.
• Full app suite — 759 passed, 1 failed. The single failure is the pre-existing HyperFrames Video Studio > keeps desktop panel titlebars draggable without swallowing control input assertion; it reproduces unchanged on current origin/main and is unrelated to this PR.

No new runtime dependency, route, table, persistent state, generated artifact, or credential is included.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve the metadata key read by message timing UI

When a snapshot message has time.created or time.completed, this writes those values under metadata.ipollowork, but getMessageCreated and getMessageCompleted still exclusively read metadata.opencode in apps/app/src/components/chat/utils.ts:115-123. Consequently mapped conversation messages lose their visible timestamps and assistant process durations; update the consumer alongside this namespace migration or retain the existing key.

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