brainstorming D2: investigate a structured phase-status vocabulary for spec refinement

[SUaDtL]

Spun out of #597 (D2).

Brainstorming's phase gates close in prose; a compact status contract (e.g. FRAMED / SHAPED / REFINED / WRITTEN / APPROVED-or-BLOCKED) would let /sprint and executing-plans branch on structured state instead of parsing text.

Investigate:

• Whether any consumer would actually branch on it today — if sprint/executing-plans have no mechanical reader, the vocabulary is decoration; find the reader first.
• Where the status lives: the spec file's frontmatter, the sprint log, or a gate-events line (which already has an append-only schema).
• Cross-skill scope: if adopted, tdd/commit-gate phase boundaries have the same shape — decide whether this is a brainstorming feature or a house convention, and if the latter, route it through an ADR.
• Relation to Make INDEX/routing-surface consistency a CI-checked invariant, not an authoring discipline #592 (routing-surface consistency as CI-checked invariant): a status vocabulary is another surface that can drift from its consumers.

Done when: a reader is identified and the contract ships with that reader, or the idea is declined with the reason recorded.