feat: add repl command layer

[rlippmann]

What changed

• Added REPL command-layer commands on top of controller APIs: state, preview , and step .
• Kept bare REPL input behavior unchanged.
• Routed REPL progression and preview through controller exports to preserve engine authority.
• Allowed read-only observational commands during pending clarification while still blocking non-confirmation mutating progression.
• Removed explain alias to keep the command surface small.
• Added and updated REPL tests for command parsing, pending behavior, and preview non-mutation behavior.
• Added concise README API coverage for public controller exports and decision-kind constants.

Why this was needed

• Establishes the 0.7 REPL/controller command semantics before JSON-mode output.
• Hardens boundaries so hosts use stable controller/runtime queries instead of checkpoint internals.
• Keeps a minimal, explicit command surface for predictable scripting and future TS parity.

Closes #84

[rlippmann]

Added a small follow-up test coverage pass for user-facing REPL behavior:

• step no while pending clarification
• interactive preview while pending
• interactive help while pending
• idempotent preview reports would_mutate: no

[rlippmann]

Added targeted contract tests to fix the REPL coverage gate failure.

Result: src/context_compiler/repl.py now reaches 100% coverage in the contract slice (pytest -m "contract" --cov=context_compiler.repl --cov-fail-under=100).