feat(tui): expose scrollToMessage to TUI plugins - #46047

Issue for this PR

Related: #37699 (scrollbar message-boundary markers). No separate issue was filed for this; the motivation is the same "find an earlier message in a long session" problem.

Type of change

• New feature

What does this PR do?

Adds scrollToMessage(messageID) to the TUI plugin state API, letting a plugin scroll the session chat viewport to a specific message.

There was previously no way for a plugin to do this: the session scrollbox is a module-local ScrollBoxRenderable inside the Session route, and the only built-in way to jump to a message is the ctrl+x g timeline dialog. This PR:

• adds a small sessionScroll controller (packages/tui/src/util/session-scroll.ts) that holds the currently displayed session's scrollbox,
• attaches/detaches it from the Session route, mirroring the existing toBottom setTimeout-deferred pattern so the ref is assigned before attach,
• exposes it on the plugin state API as scrollToMessage(messageID): boolean (returns false when no session is attached),
• factors the four existing "find child by id, then scrollBy(child.y - scroll.y - 1)" blocks into one shared scrollToMessageID helper in util/scroll.ts.

The plugin-facing types (packages/plugin/src/tui.ts) and the test fixture (packages/opencode/test/fixture/tui-plugin.ts) are updated, and the method is documented in specs/tui-plugins.md.

How did you verify your code works?

• bun run typecheck passes (30/30 tasks).
• oxlint clean on the changed files.
• Ran bun dev and confirmed through a temporary home_bottom probe that typeof api.state.scrollToMessage === "function".
• Built a small TUI plugin that registers a sidebar_content slot listing user messages and calls scrollToMessage on hover/click; confirmed the sidebar renders and the method is callable.

I did not verify the resulting scroll position pixel-perfectly beyond the above (headless terminal); the scroll math is unchanged from the existing timeline/fork call sites, which now share the same helper.

Screenshots / recordings

None — terminal UI change, not meaningful as a static screenshot.

Checklist

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