Add a `design` skill: guard a deliberate separation with a conformance check instead of collapsing it

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Problem

When a design decision would put different things in their right places (separate concerns, split machine-truth from human-prose, keep a pattern's pieces in distinct homes), the fear that those pieces will drift out of sync is often treated as a reason to collapse the design. But drift is guardable: a conformance check that fails on divergence lets you keep the clean separation and the safety.

This design-decision heuristic is only partially present in the corpus today. engineering-practices carries the tactical mechanics — "keep a single source of truth … and have a test fail if it drifts" (line 11), and "point to B, don't paraphrase B" (line 12). What's missing is the higher-altitude decision reframe and one specific nugget:

• Drift-fear is not a veto on a correct separation — it's a prompt to write the guard.
• The guard ships in the pack/module that defines the pattern, so enforcement travels with the design decision wherever the pattern is reused.

Decision

Owner chose a new skill (over extending engineering-practices or adding to checks/DESIGN.md) as the home, to seed a place for design-decision judgment to accumulate.

Scope

• Add skills/design/SKILL.md — the decision heuristic, a short decision sequence, the boundary against "duplicate freely," and one corpus-grounded example (the pack-declaration design: machine-truth in .claudinite-checks.json + hook-rendered prose, both from one declaration, guarded by the fingerprint check).
• Cross-link engineering-practices for the drift-guard mechanics rather than duplicating them (honors the corpus's own "point to B" rule).
• Add the catalog row in skills/README.md.
• No per-skill wiring needed — bootstrap Part 7's structural loop auto-mounts new skill dirs on re-bootstrap.

Not in scope: migrating existing engineering-practices bullets into the new skill (larger, reference-touching change; revisit as follow-up if the design skill grows).

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Resolved differently than the title proposes: no design skill. On review, the guideline is a sharpening of engineering-practices line 11 (single source of truth + drift guards), and its real trigger is "writing or editing code" — which that skill already owns. A broad design trigger would fire at the wrong moments and miss the instant the rule applies (about to duplicate a fact).

Landed in PR #148 as a one-line sharpening of line 11: one definitive place per fact; split only when a technological constraint forbids a single home, and then the drift-guard test must document the places it watches and the reason for the split.

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