ci: build the docsite on every pull request

[rqbazan]

Summary

Every pull request builds the docsite, without deploying it. The SEO audit still runs by hand, and production deploys are unchanged.

 Docsite
+  Build       on pull_request
   Audit SEO   on workflow_dispatch

Evidence

• Before: no PR check builds the docsite.
  After: this PR shows a "Build" check from the Docsite workflow.

Merge Danger

Door: two-way

Blast Radius: CI-only. "Build" is not a required check.

🤖 Generated with Claude Code

https://claude.ai/code/session_017sXwJhk5oRMrzxnA1zWxER

[changeset-bot]

⚠️ No Changeset found

Latest commit: d74c79e

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