[ci] Set up otpot CI pipeline

[ethan-thomason]

Upstream conpot workflows (black.yml, python.yml, xmllint.yml) have been
disabled and moved to .github/workflows/disabled/ as they were failing on
every commit.

Need to set up otpot-specific CI:

• Run test suite on Python 3.10
• Run test suite on Python 3.11 (once cpppo is resolved, see [python-compat] Python 3.11 support blocked on cpppo library #2)
• Code formatting check (black)
• XML template validation (xmllint)
• Badge on README showing CI status

This should be tackled after the test suite is audited and passing on
Python 3.10 (prerequisite work).

[ethan-thomason]

Partial progress: make lint and make format now work via uv + black. Remaining work: add .github/workflows/lint.yml to run make lint on every push and PR.