Prevent negated maintainer readiness from approving strategic issues

[laclance]

Finding

The shared readiness detector considered the phrase ready for implementation an approval even when embedded in Not ready for implementation. Because positive readiness is evaluated before maintainer hold logic, an explicit refusal could be misread as permission, allowing a strategic proposal or upstream-tool issue into the queue.

Fix

• Detect explicit local negations of readiness/implementation-welcome phrases, and never treat them as a positive readiness signal.
• Classify these negated statements as an implementation hold, while preserving the existing latest-trusted-maintainer-comment-wins rule.
• Reuse the shared decision in maintainer-authored issue bodies and both recently added strategic-readiness filters; no new API requests or scanner flow changes.
• Add unit tests for negated variants, genuine approvals, comment ordering/authority, and end-to-end strategic rejection. Document the rule.

Scope

Narrow readiness correctness fix. No other audit findings addressed, no private-instance deployment, no changes to paid, discovery, scoring, state persistence, or delivery.

Validation

Requires green CI formatting/lint, strict types, coverage and Python compatibility. Remains a draft PR until explicitly authorized for merge.