Author a new mkit SPEC

mkit specs are the authoritative source for on-disk formats, wire formats, and subsystem behavior. Anything that mutates an on-disk or wire format requires a matching docs/specs/SPEC-*.md change (CONTRIBUTING review bar item 5). Work in the checkout, not from memory — the specs evolve, so confirm the current conventions before you write.

Ground truth to read first (they win over anything here if they have changed):

• docs/specs/README.md — the spec index you will register into.
• docs/STYLE-GUIDE.md — the writing conventions specs follow.
• Two model specs of the shape you are writing: docs/specs/SPEC-OBJECTS.md (a byte-layout format spec) and docs/specs/SPEC-WORKTREE.md (a subsystem spec). Skim one more neighbor if your topic differs.
• docs/PARITY.md — only if your spec introduces or changes an in-scope command or behavior (see step 3).

Step 1 — Scaffold the spec file