website: Take released docs from a website/<tag> branch when present

[huacnlee]

Description

The v0.7.0 website still shows gpui-kit = "0.6". The cause is the content, not the build cache: the latest release is rebuilt on every run from the tag's docs and homepage, and v0.7.0 was tagged before #3284 replaced the hardcoded versions with {{gpui_kit_version}}. A published tag should not move, and correcting its docs should not require a new release.

script/build-website-versions now fetches website/* branches. When origin/website/<tag> exists, the docs and HomeApp.vue are snapshotted from it instead of the tag. GPUI_KIT_VERSION and GPUI_PRE_VERSION still resolve from the tag. The historical build cache key now includes the content commit, so an updated branch is rebuilt instead of served from cache.

The website/v0.7.0 branch is already pushed. It is cut from v0.7.0 and carries only the content part of adde487 (#3284): 19 docs files plus HomeApp.vue, identical line-for-line to that commit, with no other commits.

After merging, manually run Release Website from main. The automatic workflow_run trigger checks out the tag commit and would use the old script.

How to Test

• bash -n script/build-website-versions passed.
• script/snapshot-website . website/v0.7.0 <dir>: the snapshot has no gpui-kit = "0.6"; docs use {{gpui_kit_version}} and HomeApp.vue uses props.kitVersion.
• The full versioned build was not run locally.

Checklist

• Read CONTRIBUTING.md and followed its guidelines.
• AI-assisted build-script change, reviewed.

🤖 Generated with Claude Code

[huacnlee]

Not merging: the script change is only needed once to republish v0.7.0. Release Website is run from this branch via workflow_dispatch instead.