DEN-763: stop non-code paths from triggering the Maven build

[vikshab]

Ticket

Changes

• Added paths-ignore to the push and pull_request triggers in maven.yml
• Bumped reviewdog/action-markdownlint from v0.26.2 to v0.31.2 in reviewdog.yml

Background
The dpe-agent-config sync and agent-config files touch only markdown and config paths, which do not need the 3-profile Maven build.

TODOs

• Confirm this PR runs the build, since maven.yml itself is not ignored
• Confirm the markdownlint job passes

[Adam-Vollrath]

Small, low-risk, well-scoped change — added paths-ignore to maven.yml's push/pull_request triggers so markdown/agent-config-only changes skip the 3-profile Maven build. No prior comments/reviews.

Answering the PR's own open TODO ("Confirm this PR runs the build, since maven.yml itself is not ignored"): confirmed — maven.yml is not in its own ignore list, so a change to the workflow file itself still triggers it, and indeed build (druid-30.0.1, 17) / build (druid-34.0.0, 17) / build (druid-35.0.1, 21) all ran and passed on this PR. That TODO can be checked off.

Checked the one real risk with this pattern: a paths-ignore filter can cause a required GitHub status check to simply never fire for a matching PR, permanently blocking merge if branch protection requires it. Checked directly — main has no branch protection rule and no ruleset configured on this repo (GET .../branches/main/protection → 404 "Branch not protected", .../rulesets → empty), so there's no required-check enforcement that could get stuck. This change is safe as-is.

Forward-looking scoping, not a bug: this repo hasn't been onboarded to dpe-agent-config yet (no .dpe-agent-config/, .claude/, .agents/, AGENTS.md, etc. on the base branch), but the ignore list already includes all of those paths — sensible sequencing so the eventual onboarding sync PR also skips the build, rather than needing a second follow-up PR.

The markdownlint check failure is unrelated CI flakiness, not caused by this change: the job's Docker build for reviewdog/action-markdownlint fails on apt-get update hitting 404s against deb.debian.org's security mirror — nothing about this PR's content. actionlint and yamllint both pass, confirming the new YAML is syntactically valid.

One non-blocking nit: .github/**/*.md (recursive) is redundant with the top-level *.md entry for anything at repo root, and I'm not fully certain GitHub Actions' path-glob treats ** as zero-or-more directories vs one-or-more here — worst case a .github-nested .md file still triggers the build unnecessarily, which is just a minor extra build, not a correctness problem.

Approving.