ci: build pull request RPC docs from the event commit

[djh58]

Summary

The pull request build of the development RPC docs can fail at "Validate version source checkout" with no code problem. #225 hit it: Checked out 967241a7…, expected 792c3c1e….

• Cause. For a pull request, the build job checks out refs/pull/<n>/merge, then requires it to be the event's merge commit (github.sha). GitHub moves that ref whenever the base branch or the pull request's stack changes. A run that overlaps such a change checks out a newer merge commit and fails.
• Fix. Check out the event's merge commit by hash, as the release entries already do. The equality check stays, so it now always holds for the commit the event named. The unused --development-checkout-ref plumbing is removed from the workflow and from doc/rpc/versioned_site.py.

Testing

• Ran focused unit or functional tests for the changed area: python3 -m unittest discover -s test/rpc_docs (27 tests, 1 skipped because MkDocs isn't installed locally). The build-plan test now asserts that the development entry checks out its resolved commit.
• Ran lint or formatting checks relevant to this change: actionlint on rpc-docs-pages.yml.
• Not run. Reason: the GitHub-hosted build runs on this PR, because it changes the workflow.

Target Branch

• This PR targets a maintainer-requested release branch: 1.x.x, the only branch with the versioned RPC docs workflow.

Risk / Review Notes

• Consensus, script, crypto, wallet, P2P, release, CI, or security-sensitive behavior changed.

Notes: CI only. Pushes to main, releases and manual runs are unchanged; they already checked out by hash.

Docs / Process Impact

• No docs change needed.

🤖 Generated with Claude Code

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