[ci] Do not skip full CI solely from a docs-prefixed PR title

[ting-hong-shieh]

Symptom

A pull request whose title starts with docs skips every substantive CI job, even when the diff contains Rust or Python code.

.github/workflows/ci.yml currently:

• forces full_ci to false when startsWith(github.event.pull_request.title, 'docs');
• skips the path-classification step under the same condition; and
• allows the aggregate CI Success job to pass when its dependencies are skipped, because it fails only on failure or cancelled.

A mixed or code-only PR titled docs: ... can therefore satisfy the required aggregate check without running lint, SPDX validation, Rust checks, mypy, Python tests, or the install smoke test.

Reproduction

1. Open a pull request that changes a Rust or Python source file.
2. Give it a title beginning with docs, for example docs: update parser behavior.
3. Observe that Detect non-documentation changes is skipped and full_ci resolves to false.
4. Observe that the substantive jobs are skipped while CI Success succeeds.

This is based on workflow expression evaluation; no provider credentials or external services are involved.

Expected vs. actual

• Expected: Whether full CI runs is determined from changed paths. A title may describe a PR, but it should not override a code-bearing diff.
• Actual: The docs title prefix bypasses path detection and all code gates.

Proposed fix

Always run dorny/paths-filter for pull requests and derive full_ci from changed paths only. If the title shortcut was added for a narrower generated-docs workflow, scope that behavior to the relevant paths instead of the PR title.

Environment

• Workflow: .github/workflows/ci.yml on b256d936f1d77bf13ec9bec399ea0a253e07ca05
• Event: pull_request

Additional context

This conflicts with CONTRIBUTING.md, which says all code must pass the listed gates. PR #126 introduced path-based documentation-only skipping; PR #254 later added the title shortcut. Neither tracks a current fix.

I searched open and closed issues and pull requests for docs prefix skip CI, startsWith pull_request.title, docs-only title full_ci, Detect code changes docs title, and full_ci docs; I found no matching report or in-flight fix.

I'm happy to send the small workflow patch if maintainers agree that paths should be authoritative.