skills(platform): delete the placeholder eval README and drop its ceiling row (#14681)

The file was a stub for evals that were never written: it says the directory
is "reserved for future skill evaluation tests", that "When implemented, evals
will follow this structure", and that it is "Not yet implemented — This is a
placeholder for future development". The tree it prints names ten eval files,
none of which exists anywhere in the repository.

Nothing read the path. Repo-wide, the single reference to
skills/objectstack-platform/evals was its own ceiling row in the token ratchet,
so the row goes with the file and the package ceiling steps down by 514 tokens.
No other row is touched, no ceiling is raised, and no prose changes.

Verified before the deletion (the ruling keeps real fixtures, so the directory
had to be proven fixture-free): the evals directory held README.md and nothing
else, with zero subdirectories.

Reverse verification: with the file deleted and the row still present,
check-skills-token-ratchet exits 1 with "cannot read
skills/objectstack-platform/evals/README.md — red, not a skip (#4690)"; with
the row dropped it exits 0 over 30 authored bundle files; --self-test passes
64 cases.

Ruled A for item 2 of the eval-stub decision card (#14296): planned-eval stubs
are deleted, their ceilings step down, real fixtures stay, no new eval packages.


Claude-Session: https://claude.ai/code/session_01LraLgQVGq8egUwfYZpbYt1

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-litant

scripts/check-skills-token-ratchet.mjs

@@ -427,7 +427,6 @@ export const CEILINGS = new Map([
   ['skills/objectstack-data/rules/validation.md', 3109],
 
   // objectstack-platform
-  ['skills/objectstack-platform/evals/README.md', 514],
   ['skills/objectstack-platform/references/plugin-hooks.md', 2628],
   ['skills/objectstack-platform/rules/plugin-lifecycle.md', 2408],
   ['skills/objectstack-platform/rules/service-registry.md', 2331],