docs: address Copilot review round 9 on #2203

Restrict the truncation guard to PLAIN scalars. `yaml` attaches a legitimate
trailing comment to quoted and block scalars too, so keying on `.comment` alone
rejected valid frontmatter — `description: "Complete value" # note` was reported
as truncated, telling the author to quote a value that was already quoted. The
regex this replaced tested the same thing by reading the first character; moving
to the node dropped the check rather than encoding it. Verified all three
shapes (double-quoted, single-quoted, block) against the parser, and added the
regression cases.

Also correct the `--max-turns 1` bullet in docs/skill-authoring.md, which
claimed the skill must be the model's first tool CALL. `collectSkillInvocations`
scans every tool_use block in the assistant event without regard to order, so a
turn calling Read and Skill together scores a hit. What actually loses the
sample is needing a tool RESULT first — that decision lands in turn two, which
never runs. The practical advice is unchanged; the mechanism now matches the
collector.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SyBPs7caoMcyvnp9ArMBhV
Signed-off-by: cliffhall <cliff@futurescale.com>

docs/skill-authoring.md

@@ -16,9 +16,15 @@ the `Skill` tool fired with the expected name.
 
 Four properties of that harness drive everything below:
 
-- **`--max-turns 1`.** The skill must be the model's **first** tool call. If it
-  opens a file first — even on its way to a perfect answer — the sample is a
-  miss. This is the single most important fact about writing cases.
+- **`--max-turns 1`.** The skill must fire in the model's **first assistant
+  turn**. Ordering _within_ that turn does not matter — `collectSkillInvocations`
+  scans every `tool_use` block in the event, so a turn that calls `Read` and
+  `Skill` together still scores a hit. What loses the sample is needing a tool
+  *result* first: read a file, look at what came back, then decide — that
+  decision lands in turn two, which never runs. So a prompt that invites the
+  model to go look at something before answering is a miss even when its
+  eventual answer would have been perfect. This is the single most important
+  fact about writing cases.
 - **The session is fresh, but not empty.** `CLAUDE.md` → `AGENTS.md` is loaded
   in full on every turn, so anything the rules file already answers gets
   answered without a skill.