docs: clarify 0.8 documentation direction

[rlippmann]

What changed

• document the approved 0.8 direction in the milestone and architecture docs, including the authority / acquisition / composition framing and the non-authoritative boundary for directive drafting
• add the 0.9 candidate planning note for canonical export integrity and hashing, with explicit non-goals around signing and security guarantees
• strengthen contributor and agent guidance so documentation, examples, and fixtures are treated as contract material rather than optional commentary
• reorganize the README so library integration appears before REPL usage, frame the REPL as an interactive playground, and separate machine-readable --json usage from interactive REPL guidance
• polish README section naming and remove duplicated --with-preprocessor explanation in the machine-readable CLI section

Why

• the repository needed the approved 0.8 and 0.9 documentation direction recorded before any implementation planning or split work
• the architecture and governance docs needed clearer boundaries around what remains authoritative, what belongs in drafting layers, and how agents should handle disagreements between specs, fixtures, docs, and implementation
• the README needed to reflect the intended positioning of Context Compiler as a library/API first, with the REPL presented as a playground and debugging tool rather than the primary integration path

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)