feat(index): generate the skills table from frontmatter

Second half of #29. The index was hand-maintained and had already drifted
once: dream and experiment-spec shipped with no row, their rows lost to a
squash-merge after a directory reorg. bin/check-index caught that after the
fact; generating removes the class.

The curated summary and trigger move into each SKILL.md as `summary:` and
`use_when:`, carried over verbatim from the existing table rather than derived
from `description:` — description is a paragraph written for model selection,
and truncating it would have produced a worse table than the one it replaced.
Knowledge now lives once, next to the skill it describes, which is the argument
that made a hand-maintained index the wrong shape to begin with.

The table is written between markers, so the wiring docs, the relationships
tree and the trigger table below it are untouched — those are prose, not data.
Rows are alphabetical now; the conceptual grouping the old order carried lives
in the relationships tree, which is the better place for it. A fourth column
shows each skill's surfaces, which is what makes #29's field visible rather
than merely enforced.

bin/check-index and its test are deleted: a generated table cannot drift in
either direction, so `generate-index --check` subsumes both of its assertions.
Verified by mutation — deleting the tdd row turns `task check` red with
"SKILLS_INDEX.md is out of date".

Closes #29
Bump-Type: minor

skills/grill-me/SKILL.md

@@ -1,6 +1,8 @@
 ---
 name: grill-me
 surfaces: [claude-code, library, chat]
+summary: "Structured plan interrogation — Quick Poke, Full Grill, Pre-mortem"
+use_when: "Stress-testing a plan before committing; end of Diamond 1; before promoting to pre-prod"
 description: Critically interrogates plans, ideas, and proposals to make them sharper, more robust, and better grounded — with the explicit goal of producing a meaningfully better plan by the end. Use this skill whenever the user wants to stress-test a plan, prepare for hard questions, check if an idea is ready to act on, validate a hypothesis, or explicitly asks to be challenged, questioned, or grilled. Also trigger when a conversation has been exploring an idea for a while and it's time to converge — use this skill to check if the idea is actually ready. Trigger phrases include "grill me", "challenge this", "poke holes", "am I missing something", "is this ready", "stress-test this", "devil's advocate", "what am I not seeing".
 ---