docs(loop): add implementation checkpoint ledger (STATUS-001)

[rfreel]

Motivation

• Provide a persistent checkpoint ledger to record one-milestone cycle evidence and meet the repo's cycle workflow requirements.
• Improve traceability for cycle acceptance criteria, commands run, key outputs, deviations, and next actions.
• Make the checkpointing expectation discoverable by adding guidance to the repository README.md.

Description

• Add IMPLEMENTATION_STATUS.md containing Cycle 2026-02-12 / STATUS-001 with acceptance criteria, test plan, commands executed with exit codes, key outputs, deviations, and next action.
• Update prd.json to include the STATUS-001 milestone and mark it as passed with a short note referencing the new ledger.
• Update README.md to add a ## Checkpointing section that instructs users to update IMPLEMENTATION_STATUS.md after checks pass.
• Append a progress line to progress.txt and add a new [OK] entry to loop/log.txt after running the loop script.

Testing

• Ran python -m json.tool prd.json which exited 0 and confirmed prd.json is valid JSON.
• Ran bash scripts/ralph/radius.sh loop/log.txt 50 which exited 0 and enforced the log radius as expected.
• Ran bash scripts/ralph/loop.sh which exited 0, exercised the canonical loop, and wrote a new [OK] line to loop/log.txt.
• Attempted python -m pip install --user pre-commit which exited 1 due to package index/proxy restrictions in the environment, so a local pre-commit run --all-files was not executed locally (CI remains the tangent for back-pressure).

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Codex Task