Plan CTX factory guardrails and task contracts

[Garabed96]

What changed

• Adds a planning doc for CTX factory guardrails, plan task interfaces, progress ledgers, and simplification checkpoints.
• Captures the intended workflow changes before implementation so they can be reviewed and iterated later.

Why

Factory iteration and long-running plan execution need stronger guardrails to reduce token waste, code drift, and compaction-related task replay.

Notes

• This PR is intentionally docs-only and draft.
• It does not implement the changes yet.
• The plan is project-generic and scoped to ctx-plugin itself.

Validation

• Not run; docs/planning change only.

Follow-up

• Review the open questions in the planning doc.
• Convert approved parts into implementation tasks across both Claude and Codex plugin trees.