Fix API docs overlay workflow checkout

[llewellyn-sl]

Summary

• isolate the sparse API version-check checkout under docs-version-check so the main checkout starts from a normal workspace
• keep the duplicate-version check pointed at the sparse checkout path
• pass the pinned Speakeasy version to the installer process so it does not drift to latest

Root cause

PR #1119 introduced a sparse checkout for the early version check and then reused the same workspace for the full checkout. The later workflow step expected .claude/skills/api-overlay-generator to exist in the root checkout, but the failed July run showed that path was absent from the runner worktree even though it existed in the commit. The later skill rename PR updated paths correctly; it changed the error path but likely did not introduce the underlying checkout issue.

Validation

• git diff --check -- .github/workflows/generate-openapi-overlays.yml
• pre-commit during commit: check yaml passed
• shell sanity check confirmed VERSION reaches sh for the Speakeasy installer

Note: actionlint still reports pre-existing shellcheck warnings in unrelated workflow steps.

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