feat: remove scope inheritance, add do-block form for scope options

[C-Sinclair]

Summary

Removes inherits from the scope DSL and enables a do-block form for setting description (and any future scope option):

scope :my_scope_name, expr(...) do
  description "This is important"
end

Breaking change: inherits is gone. The old positional form (scope :own_draft, [:own], expr(status == :draft)) and the keyword form (scope :name, expr(...), inherits: [:parent]) no longer compile. Write each scope as one expression:

# Before
scope :own, expr(author_id == ^actor(:id))
scope :own_draft, [:own], expr(status == :draft)

# After
scope :own, expr(author_id == ^actor(:id))
scope :own_draft, expr(author_id == ^actor(:id) and status == :draft)

Why: scope inheritance added hidden coupling (child behaviour depended on parent filters that might live in a different module), interacted awkwardly with write: and argument-based scopes, and — because inherits was an optional positional arg — blocked scope from supporting Spark do-blocks at all. Inlining with and is mechanically trivial, reads plainly, and shows up straight in AshGrant.explain/4.

Domain-level scope merging is unchanged — a resource still sees scopes defined on its domain. Only scope-to-scope inheritance is removed.

Net change

Net −191 LoC across lib/. Info's resolve_scope_filter/3 is now a direct lookup; AddArgumentResolvers lost the recursive DSL-state walker.

Docs

• usage-rules.md: scope entity section updated, inheritance rule replaced with "combine inline" DO.
• README.md, guides/scopes.md, guides/authorization-patterns.md, guides/scope-naming-convention.md, guides/getting-started.md, guides/advanced-patterns.md: removed inheritance references, updated examples.
• guides/migration.md: new section explaining the removal and migration path.
• CHANGELOG.md (Unreleased): breaking change + added do-block form.

Test plan

• mix compile --warnings-as-errors clean
• mix test — 1045 tests pass (1 pre-existing failure in default_field_policies_test.exs, unrelated)
• mix format --check-formatted clean
• mix credo — only pre-existing non-blocking suggestions
• Test resources and guide examples all compile after inlining combined filters
• Do-block form verified via new tests in test/ash_grant/scope_dsl_test.exs