build: add GitHub Actions CI workflows

[icosta-sifive]

Summary

• Add unit.yml: runs make check on ubuntu-24.04 and macos-15 on push/PR to main.
  Runners are pinned (not *-latest) to avoid silent breakage when GitHub shifts a runner OS.
• Add pr-checks.yml: three PR-only jobs:
  • checkpatch — runs checkpatch.pl --no-tree over all non-merge commits; covers DCO
    (Signed-off-by presence/correctness) and kernel-style code hygiene. Replaces a hand-rolled
    grep-based DCO check. FILE_PATH_CHANGES is suppressed (kernel MAINTAINERS convention,
    not applicable here).
  • commit-format — validates that commit subjects start with one of the project's required
    prefixes: lib: include: test: docs: debian: build:. checkpatch.pl has no
    concept of project-specific prefixes so this check is kept separate.
  • docs — runs make docs only when docs/** or include/** files change, gated via
    dorny/paths-filter.
• Add htmldocs Makefile target: runs doxygen without the LaTeX/PDF step. The existing docs
  target depends on htmldocs so the full PDF build is preserved for local use. CI uses
  htmldocs to avoid requiring pdflatex on the runner.

Notes

• The project uses Linux kernel commit style, not Conventional Commits — off-the-shelf
  commitlint would reject valid commits, hence the custom prefix check.
• checkpatch.pl is downloaded fresh from torvalds/linux at CI runtime (no vendored copy).

Test plan

• Push a commit missing Signed-off-by: — checkpatch job should fail

• Push a commit with a subject that lacks a required prefix — commit-format job should fail

• Push a docs change — docs job should run and pass

• Push a non-docs change — docs job should be skipped

• All jobs green on a well-formed PR

[icosta-sifive]

@pathakraul in order to make this work, some settings need to be done on the repo:

1. Go to https://github.com/icosta-sifive/librpmi/settings/branches
2. Click Add branch protection rule
3. Branch name pattern: main
4. Check Require status checks to pass before merging
5. Search for and add the job names:

• unit-test
• checkpatch
• commit-format
• docs

6. Optionally check Require branches to be up to date before merging
7. Click Save changes