Add configurable array merge strategies to Helm value coalescing

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What this task asks for

Add chart-scoped append and key-based merge strategies for coalescing arrays during Helm value merging. Strategy-aware global values: when a subchart declares a strategy for a path prefixed with `global.`, that strategy applies when global values are merged into the subchart's scope. Merge strategy annotation warnings must be emitted by the same lint rule that validates other Chart.yaml fields (name, version, type, dependencies). Strategies must be applied to user values and chart default values at the per-chart coalescing level, so that annotated arrays are pre-merged before individual keys are processed by the existing coalescing logic. Chart arrays must be deep-copied before strategy application to avoid mutating chart defaults. The chart accessor interface must expose annotations from chart metadata. Strategy extraction must return only actionable strategies: entries with `"merge"` that lack a companion merge-key are returned as `"append"`, and annotations with empty or invalid paths are excluded.