chore: remove bundled preprocessor surfaces

[rlippmann]

What changed

• removed the bundled experimental preprocessor package tree and REPL preprocessing flag
• deleted core-owned integration examples that now belong outside this repository
• removed acquisition-layer tests and fixtures while keeping authority-layer engine, controller, checkpoint, preview/diff, and grammar coverage intact
• cleaned up packaging, lockfile entries, and CI references for the deleted extras and example surfaces

Why

• core no longer owns acquisition-layer drafting or bundled integration example surfaces
• this keeps context-compiler scoped to deterministic authority-layer behavior only

Checklist

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