fix(opencode): name the release when upgrading OpenCode

Since opencode 1.18.x, `POST /global/upgrade` requires a `target` semver in
the body. OpenChamber sent an empty object, so every "Update OpenCode" click
came back 400. The rejection arrives as `{name, data:{message}}`, which has
no `error` field, so the user was left with the bare status text: "Bad
Request".

Resolve the target from the latest release — the same lookup the upgrade
prompt already uses to decide there is anything to offer — and fail with an
explicit code when it cannot be resolved, rather than sending a body opencode
is guaranteed to reject. Read the upstream rejection message so a refused
upgrade explains itself.

The VS Code extension carries its own copy of this flow and had the same two
defects; both are fixed there.

fixes #3121

packages/vscode/CHANGELOG.md

@@ -3,6 +3,7 @@
 - Picking a remote branch such as `origin/main` in the Git branch selector now switches you to that branch instead of leaving the repository on a detached `HEAD` with no branch name.
 - GitHub Copilot usage now shows a single AI Credits window, matching Copilot's token-based quota, in place of the old Chat Requests and Completions windows (thanks to @jakoss).
 - The context usage readout now reports the session cost including everything its subagents spent, matching the work status panel instead of showing a lower figure.
+- Updating OpenCode no longer fails with a bare "Bad Request": the extension names the release to install, which recent OpenCode versions require, and shows OpenCode's own reason when an update is refused.
 
 ## [1.21.0] - 2026-08-26