docs: address Copilot review round 3 on #2203

Replaces the last `project-structure` case that `AGENTS.md` answers — a
helper used by the CLI and TUI belongs in `core/` per its own directory
tree. This was also the only case below 100% in the previous suite, so
the static reading and the score agreed. The replacement targets the
skill's first stated purpose, locating code, which `AGENTS.md` gives no
method for; it probed 3/3 and now measures 100%.

Extends the truncation guard's tests to `when_to_use`. The guard loops
over both fields but only `description` was pinned, so dropping the
second from that list would have gone unnoticed. The new case pairs a
quoted description with an unquoted `when_to_use` and asserts the
description is not implicated.

Scopes the negative-case requirement in the prose to model-invoked
skills; round 2 scoped the checklist but left this sentence saying
"every skill file".

Refreshes the stale `~2.8k of 4k` listing figure. The other instance is
past-tense about the three-to-nine experiment and stays, now said
explicitly so two numbers do not look like a disagreement.

Records the eval's real blind spot: all three cases review identified as
`AGENTS.md`-answerable were scoring 80-100% when flagged, so a high score
does not clear a case of overlap. The eval can show a case failing; it
cannot show a passing case is well-aimed.

RUNS=5 skills:eval: 63/63. local:gate passes, 34/34.

Signed-off-by: cliffhall <cliff@futurescale.com>

.claude/skills/project-structure/evals/evals.json

@@ -16,7 +16,7 @@
     "expect": "project-structure"
   },
   {
-    "prompt": "Does a helper that only the CLI and TUI use belong in core or in one of the clients?",
+    "prompt": "I need to find where a feature is implemented but I do not know which client owns it. How do I work that out in this repo?",
     "expect": "project-structure"
   },
   {