Split grill-with-docs into grilling + domain-modeling primitives

[bracyw]

What

Our grill-with-docs skill is a monolith equal to two upstream (Matt Pocock) skills glued together, pre-split: the interview half is now upstream grilling, the doc-discipline half is upstream domain-modeling. Adopt the split so both halves become reusable primitives, and fix a regression the monolith carries.

The bug

Our grilling half still has the blanket line "if a question can be answered by exploring the codebase, explore the codebase instead" — the exact wording upstream flagged as letting a grilling agent answer its own decisions and race ahead instead of asking the human.

Scope

• Add skills grilling (interview primitive, with facts-vs-decisions split + confirmation gate) and domain-modeling (glossary/ADR discipline).
• Move CONTEXT-FORMAT.md and ADR-FORMAT.md from grill-with-docs into domain-modeling; repoint the two links in improve-codebase-architecture.
• Shrink grill-with-docs to the orchestrator that runs grilling using domain-modeling; add disable-model-invocation.
• Record the decision in a new ADR-0004; add ADR pointers in CLAUDE.md.

Acceptance criteria

• grilling and domain-modeling skills load; grill-with-docs delegates to them
• No dangling references to the moved format files
• grilling no longer contains the self-grilling blanket line
• ADR-0004 added

Implemented on branch claude/start-up-ixwx20.