feat(verify): honour a disabled tester role so the LLM tester is no longer the fallback for a failed verification (AGT-4679)

[unohee]

Summary

• runTesterWithVerification takes llmFallback. With false, a deterministic infrastructure failure is rethrown (the attempt ends as infra_error) and a repository with no deterministic command gets a passing result that says nothing was verified. The LLM tester is never asked.
• PairPipeline.runTester passes llmFallback: roles.tester.enabled !== false, so only an explicit enabled: false turns it off. Default behaviour is unchanged.
• The stage log line says LLM tester is disabled, no verdict instead of falling back to LLM tester in that case.

Why

tester.enabled: false was already configured, but every verification that hit the 300 s pytest cap still fell back to the LLM tester (tester stages of 764 s and 814 s observed). The disabled role was not honoured.

Tests

• deterministicTester.test.ts: no command + llmFallback: false returns the unverified result and never calls the fallback; the default still calls it.
• pairPipeline.verify.test.ts: infra failure with the tester role disabled ends as infra_error without runTester; no command with the role disabled passes without runTester.
• Mutation checks: removing the rethrow or the unverified return makes the matching tests fail.