fix: handle negated pull request conflict guidance

[CoderDeltaLAN]

Summary:

• Fix conflict detection so negated pull request guidance is not treated as a pull request requirement.

Scope:

• Updates deterministic conflict matching for negated PR boundary wording.
• Adds regression coverage for negated PR guidance.
• Updates CHANGELOG under Unreleased.

Validation:

• python -m unittest tests.test_conflicts: passed.
• ./scripts/check.sh: passed.
• ./scripts/post-release-audit.sh from clean tree: passed.
• Pre-push diff limited to CHANGELOG.md, src/agent_rules_kit/conflicts.py, and tests/test_conflicts.py.

Risk:

• Low. Localized deterministic regex/polarity change with targeted regression tests.

Rollback:

• Revert the PR commit.