CI: coverage reporting + non-regression floor

[ernestprovo23]

Add coverage reporting and a non-regression floor. Suite is strong on council/modes/adapters/registry/config but transport/cli/logging are untested (see the test-gaps issue).

Scope

• Add pytest-cov to dev extras.
• CI runs pytest --cov=conclave --cov-report=term-missing.
• Set --cov-fail-under=<measured baseline> — measure first, do not guess the number.
• Ratchet the floor up as test-gap work lands.

Acceptance

• CI prints coverage; a --cov-fail-under gate prevents regression below the measured baseline.

Source: pre-dev planning audit 2026-06-08 (finding C-2). Depends on CI workflow issue.

[ernestprovo23]

Done — coverage reporting + 75% non-regression floor landed in PR #25 (measured 76% at the time; now 94%).