chore: add CI and community hygiene

[abhijeet-dhumal]

Depends and blocked by #1 and #2 resp.

Hardens CI and community workflows before feature work continues.

What's included:

• Makefile with dev targets - install, lint, format, test, agent, docs, build
• dco.yaml - DCO sign-off enforcement on pull_request_target
• docker.yaml - Docker build/push gated on path filter to avoid unnecessary runs
• docs.yaml - Sphinx docs build triggered only on docs/** changes
• publish.yaml - PyPI publish workflow
• welcome-new-contributors.yaml - auto-welcome for first-time contributors
• check-pr-title.yaml
• PR template expanded with related issue link, sign-off reminder, and uv test commands

[google-oss-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from abhijeet-dhumal. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Details

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment