feat: finalize Decision API migration

[rlippmann]

What changed

• Added a strict 100% CI coverage gate for context_compiler.decision and expanded Decision contract coverage.
• Documented the 0.9 Decision boundaries: step() accepts raw input, while apply_directive() accepts only CanonicalDirective objects.
• Documented terminal SemanticErrorDecision results, ordered advisory repairs, and explicit host repair selection.
• Updated Example 08 to demonstrate both Decision API entry points, structured error fields, unchanged state after semantic errors, and explicit repair selection.
• Removed the obsolete Demo 09 continuation-oriented surface and folded its meaningful behavior into Example 08.
• Removed current pending-continuation and confirmation architecture wording while preserving historical demo records.
• Bumped the package version to 0.9.0dev13.

Why

• Align the implementation guidance, specifications, examples, conformance coverage, and CI enforcement with the completed 0.9 Decision API architecture.
• Prevent stale continuation semantics and untested Decision behavior from re-entering the public contract.

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)
• strict decision coverage reaches 100%
• markdownlint issues resolved