refactor(projection): clarify inheritance overrides and commit requirements

[Fodesu]

Context

Projection definitions currently use Inherits and Authoritative, which make two separate contracts difficult to distinguish:

• how a projection reads inherited history after a fork;
• whether a projection must successfully fold before a commit can be persisted.

This makes the lifecycle semantics of conversation history, execution state, and maintenance state harder to review.

Changes

• Rename Inherits to InheritanceOverride.
• Rename InheritPolicy to InheritancePolicy.
• Keep nil as the default, so the projection follows the owning stream domain's StreamDefinition.Inheritance.
• Replace InheritStreams with the explicit InheritNone policy while retaining InheritAll.
• Rename Authoritative to RequiredForCommit.
• Update registry validation, checkpoint handling, session writers, and built-in projections.
• Use domain defaults for conversation history and explicit InheritNone for execution and compaction state.
• Add a focused fold-level test covering default, all, and none inheritance behavior.

Validation

• go test ./agentcore/...