docs: align milestone language with 0.5 premise/policy model

[rlippmann]

What changed

• Rewrote docs/DirectiveGrammarSpec.md to the 0.5 model (schema version 2) with explicit premise/policy semantics.
• Added narrowly scoped clarifications for replacement identity, policy idempotency, clarify prompt scope, and pending-clarification precedence.
• Updated docs/DescriptionAndMilestones.md to remove M1-shape contradictions and align high-level wording with premise/policy terminology.

Why

• The docs now consistently reflect the breaking 0.5 semantic revision and remove conflicting legacy M1 behavior descriptions.