feat(tui): replace V1 auto-update with V2-style /update dialog

[thdxr]

Issue for this PR

Closes #

Type of change

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

What does this PR do?

OpenCode 1 no longer installs updates on its own. It now uses the same update flow as OpenCode 2: it checks for a new version, shows a notice, and installs only when you run /update.

Before

• On startup, patch releases were installed silently. Minor and major releases opened a blocking "Update Available" confirm dialog, and accepting it installed through the server's global.upgrade endpoint.
• The check ran once per launch.

After

• Nothing is installed automatically. upgrade() now only works out whether a newer version exists. autoupdate: false and OPENCODE_DISABLE_AUTOUPDATE still turn off checks, and true and "notify" now both just notify.
• Like the V2 CLI, the TUI process checks every 10 minutes and replays the latest result to the TUI through an updater source.
• The home screen shows V2's notice: /update to install v1.x.y. After an install it reads /exit restart to use v1.x.y. It's placed under the logo and uses the same order, wording and layout as V2.
• /update opens V2's update dialog ("Update available", "An update is available. After installing, you'll be prompted to restart OpenCode.", with Skip / Update buttons). Progress ("Installing OpenCode 1.x.y…") and errors appear inside the dialog, and when it's done it shows "Update successful! A restart is required." with a Restart button.
• OpenCode 2.0 is offered only when OpenCode 1 is already up to date. In that case the notice reads /update to upgrade to OpenCode 2.0, and the dialog (normal width) says "OpenCode 2.0 is the next major release." then, on its own line, "Read more about it: https://anoma.ly/notes/opencode-2-0" (clickable), then a blank line and "It contains some breaking changes.", with Dismiss / Upgrade. Upgrade runs Installation.upgradeMajor (the upgrade --major path). When a newer 1.x release exists, the flow is the normal OpenCode 1 update and OpenCode 2.0 isn't offered.
• As in V2, installs run locally in the CLI/TUI process using the existing Installation.method(), Installation.upgrade() and Installation.upgradeMajor(). They don't go through the server, and the server API and SDK are unchanged apart from the regenerated autoupdate description.
• Skip (OpenCode 1 updates) keeps V2's behaviour: it stores skipped_version, and the notice returns only for a newer 1.x release.
• Dismiss (OpenCode 2.0 offer) hides that notice for one week (update_dismissed_until).
• The two are independent: skipping a 1.x version never hides the 2.0 offer, and dismissing 2.0 never skips a 1.x version. /update keeps working either way.
• I updated the docs (config autoupdate, the OPENCODE_DISABLE_AUTOUPDATE description, upgrade --major, and a new /update entry) and the CLI help snapshot for --major.

How did you verify your code works?

• bun run typecheck (turbo, all packages)
• packages/tui: bun test (199 pass), including the new test/component/dialog-update.test.ts
• packages/opencode: bun test test/cli test/installation test/config, including the new test/cli/upgrade.test.ts
• oxlint on the changed files: no errors. Root bun run lint has one error that already exists on v1.
• I drove the real V1 TUI in tmux with a stubbed registry (npm_config_registry) and a stub bun that fakes the global install, so nothing was really installed. I checked both paths (a 1.x update available, and 1.x up to date with OpenCode 2 offered): the notice, the dialog, the in-dialog progress, the restart state and the post-install notice.

Screenshots / recordings

I captured screenshots locally of both paths: notice, dialog, install progress and restart prompt (see the session report).

Checklist

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

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