chore: add contributor license agreement and governance docs

Add lightweight contribution governance so future pull requests cannot
encumber the ability to ship the AGPL-3.0 core inside a closed/commercial
build of Axis.

- CLA.md: individual Contributor License Agreement (template, adapted from
  the Apache ICLA v2.0) granting Vir Sanghavi a perpetual, irrevocable
  copyright and patent license, including the right to relicense under any
  terms, including proprietary and closed-source.
- CONTRIBUTING.md: states the project is AGPL-3.0 and requires CLA agreement
  before merge, and explains how contributors signify agreement.
- .github/PULL_REQUEST_TEMPLATE.md: required CLA agreement checkbox.
- GOVERNANCE_TODO.md: manual follow-ups (retroactive CLA from Angel Garcia
  for eb6aba9; install CLA Assistant).

No functional, build, config, or test files changed. LICENSE unchanged.