[FEATURE]: Todo Sidebar with Linear integration (project-scoped issue management)

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Describe the enhancement you want to request

The problem

OpenCode currently has a per-session flat todo list (session.sql.ts → TodoTable) tied to a single chat session. When the session ends, the todos are stranded. There is no project-scoped task management surface, and no integration with external issue trackers like Linear. Users working on a multi-session feature lose their task context between sessions and have no way to sync work with their team's Linear workspace.

The enhancement

Add a Linear-style Todo / Issue management feature to OpenCode Desktop. The feature is an independent project-scoped system that can optionally collaborate with the Linear MCP server (https://mcp.linear.app/sse).

Core capabilities:

• Two-level hierarchy — L1 todos run sequentially, L2 sub-todos run in parallel. The feature does not validate ordering.
• Linear-aligned statuses — Backlog, Todo, In Progress, In Review, Done, Canceled, Duplicate.
• Priorities — Urgent, High, Medium, Low, No priority.
• Project/workspace scope — each OpenCode directory maps to one project; todos persist across sessions in that workspace, independent of any chat session.
• Linear MCP integration (optional) — a directory maps to a Linear Project; opening a directory switches the Linear context. Sync pull/push keeps local todos and Linear issues aligned. The Linear integration is an add-on; the sidebar works standalone without it.
• Agent integration — agents can auto-create / re-order / update todos as work progresses, via a thin kernel tool or through the existing Linear MCP system.

Why this is not a duplicate

• #21590 covers session-level status management (status flags on chat sessions themselves, sidebar filtering by status). This request is about a project-scoped task system that lives outside any session — todos persist across sessions and reference sessions/messages but are not sessions.
• #5971 covers a plugin API for custom sidebar panels. This request is for a first-party Todo surface, not a plugin API.

Implementation status

A complete implementation is proposed in #38082 (8 atomic commits, rebased on dev). UI screenshots and full design rationale (ADRs 0001–0007) are in the PR description.

Benefits

• Todos become first-class project-scoped work items, not stranded chat artifacts
• Users can manage AI-assisted work like a lightweight task board across sessions
• Linear users can sync their local todos with their team's Linear workspace without leaving OpenCode
• Agents gain a stable task surface to track multi-step work