miner: adopt live confidence floor when static readiness is unset

[aryansk]

Fixes #10338

Adopt a valid live confidence floor when the static manifest has no numeric readiness threshold, while preserving the existing raise-only behavior when a numeric static threshold exists. Adds regression coverage for the unset case.

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
269	1	268	0

View the top 1 failed test(s) by shortest run time

test/unit/miner-self-review-context.test.ts > live gate thresholds probe (#6487) > covers option defaults, sparse GitHub payloads, streaming manifest success, and label/date fallbacks

Stack Traces | 0.0308s run time

AssertionError: expected { gate: { readinessMinScore: 90 } } to deeply equal { gate: { readinessMinScore: 'x' } }

- Expected
+ Received

  {
    "gate": {
-     "readinessMinScore": "x",
+     "readinessMinScore": 90,
    },
  }

 ❯ test/unit/miner-self-review-context.test.ts:866:7

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