ci: shorten PR gates for docs and Valgrind

[membphis]

Summary

• add a lightweight changed-file classifier so docs-only PRs skip expensive CI jobs
• keep the PR Lua Valgrind job focused on smoke specs instead of the full Lua suite
• add a scheduled/manual full Lua Valgrind workflow for the complete suite
• on scheduled full Valgrind failures, create a deduplicated GitHub issue or append a comment to the existing open issue

Expected impact

• docs-only PRs should avoid build, Rust, Lua, OpenResty, fuzz, sanitizer, and Valgrind jobs
• code PRs still run the full PR gate set, but Lua Valgrind should finish much faster because property/exhaustive specs move out of the PR path
• scheduled full Valgrind failures stay visible through an open issue instead of only being buried in Actions history

Local validation

• git diff --check
• go run github.com/rhysd/actionlint/cmd/actionlint@latest .github/workflows/ci.yml .github/workflows/lua-valgrind-full.yml
• local changed-file classifier smoke test for docs-only, CI-file, and Rust-file changes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 0daa4dde-bbe0-46b4-9574-3017e5dd145c

📥 Commits

Reviewing files that changed from the base of the PR and between cf87fd4 and 934b1ec.

📒 Files selected for processing (2)

• .github/workflows/ci.yml
• .github/workflows/lua-valgrind-full.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/fast-pr-gates

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