fix(board): one spec for next-cost, and it is the rubric's

The board line's cost field had two specifications and one of them was this
script. `--help` prescribed a versionless `sonnet/xhigh`; the global model
rubric it points at spells the model `Sonnet 5 / xhigh`. A session following
either was correct by its own source, so the field drifted with nobody making
a mistake. Measured across a real 44-repo tree: 14 board lines carrying eight
distinct strings, and not one of them the form documented here - the only
spelling nobody used was ours.

The help now prescribes the rubric's, which is also what most lines already
carry. The parser is untouched and stays lenient: it reads to `;` or `-->` so
the rubric's spacing survives, and the fixtures still vary the spelling on
purpose to pin that. Read-side leniency was never the defect; it is what makes
a documentation-only fix sufficient.

No normalization at read, deliberately. Board lines are rewritten every session
by convention, so an unambiguous spec self-heals the existing lines in about one
session per active repo - mapping variants would be permanent code migrating
data that migrates itself. Nothing compares the field mechanically today either:
KOST is a display column and the board sorts on a numeric bucket key, so the
defect was real but latent. Two checks (28 -> 30) pin the documented form,
red first, because cleaning the data cannot fix a spec that regenerates the
divergence every session. Validation at write has no write path to attach to:
nothing in this repo emits a board line outside the fixtures.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U6EixQo6hpoRCVtiAXdnFs

CLAUDE.md

@@ -31,7 +31,7 @@ marketplace plugin. Three components, one boundary:
 - **Board (`scripts/board.sh`):** cross-repo attention board. Reads STATE.md
   next-step blocks + board lines, `git status`, and mailbox pending counts, and
   prints one line per repo. Read-only by construction: it writes to no repo, no
-  STATE.md and no mailbox. Pinned by `board-selftest.sh` (28 checks).
+  STATE.md and no mailbox. Pinned by `board-selftest.sh` (30 checks).
 
   **It lives here because the mailbox is one of its three inputs, and it carries
   the same axis distinction the mailbox does.** A pending count means *others
@@ -78,7 +78,7 @@ halves together for exactly that reason.
   builtins only in hook and tests.
 - TDD: no behavior change without a failing selftest check first.
   `bash scripts/coord-selftest.sh` must exit 0 (136/136) and
-  `bash scripts/board-selftest.sh` must exit 0 (28/28).
+  `bash scripts/board-selftest.sh` must exit 0 (30/30).
 - English for all code, docs, and commit messages (public repo). Norwegian
   trigger aliases in the skill description are deliberate.
 - Conventional Commits: `type(scope): description`.