docs: add null-narrowing and log-mocking conventions to CLAUDE.md

[kinyoklion]

Two patterns that should be applied consistently across the repo:

• Prefer Dart 3 case-pattern null narrowing (if (x case final v?)) over the ! operator. Case-patterns introduce a non-null local without runtime assertion, avoiding surprise TypeErrors.
• For tests that capture log output, use mocktail's Mock implements LDLogAdapter plus verify(...).captured rather than hand-rolling a capturing adapter. This matches the pattern used in five existing test files (validating_persistence_test.dart, hook_runner_test.dart, env_context_modifier_test.dart, operations_test.dart, anonymous_context_modifier_test.dart).

CLAUDE.md is loaded into context for every Claude Code session in this repo, so codifying these here makes them the default for both human contributors and AI agents.

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Note

Low Risk
Low risk: documentation-only changes with no runtime behavior impact.

Overview
Updates CLAUDE.md to codify two new conventions: prefer Dart 3 case-pattern null narrowing over !, and standardize log-capture in tests using mocktail (Mock implements LDLogAdapter + verify(...captureAny())) with validating_persistence_test.dart as the reference pattern.

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