feat(sdk/python): expose provider-agnostic harness variants

[ddbaron]

Summary

• expose the provider-agnostic variant option on HarnessConfig, HarnessRunner, and Agent.harness
• preserve explicit variant precedence over a #variant model suffix
• document the public option and verify its forwarding and defaults

Relationship to PR3

Related to #3, but not dependent on it.
This PR is based on main and intentionally excludes the OpenCode configuration overlay, recursion defense, and provider-specific implementation details from PR3.
The existing provider adapters already consume the resolved variant option or model suffix.

Validation

• focused harness, runner, type, and OpenCode provider tests pass
• the variant changes apply cleanly to main
• ShellCheck and whitespace checks pass

[github-actions]

Performance

SDK	Memory	Δ	Latency	Δ	Tests	Status
Python	9.0 KB	-	0.25 µs	-29%	✓	✓

✓ No regressions detected

[github-actions]

📊 Coverage gate

Thresholds from .coverage-gate.toml: per-surface ≥ 84%, aggregate ≥ 85%, max per-surface regression ≤ 1.0 pp, max aggregate regression ≤ 0.50 pp.

Surface	Current	Baseline	Δ
control-plane	87.70%	87.40%	↑ +0.30 pp	🟡
sdk-go	93.10%	92.00%	↑ +1.10 pp	🟢
sdk-python	94.33%	93.73%	↑ +0.60 pp	🟢
sdk-typescript	91.72%	90.42%	↑ +1.30 pp	🟢
web-ui	84.77%	84.79%	↓ -0.02 pp	🟡
aggregate	85.85%	85.75%	↑ +0.10 pp	🟡

✅ Gate passed

No surface regressed past the allowed threshold and the aggregate stayed above the floor.

[github-actions]

📐 Patch coverage gate

Threshold: 80% on lines this PR touches vs origin/main (from .coverage-gate.toml:thresholds.min_patch).

Surface	Touched lines	Patch coverage	Status
control-plane	0	—	➖ no changes
sdk-go	0	—	➖ no changes
sdk-python	0	—	➖ no changes
sdk-typescript	0	—	➖ no changes
web-ui	0	—	➖ no changes

✅ Patch gate passed

Every surface whose lines were touched by this PR has patch coverage at or above the threshold.