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"SKILLS_INDEX.md is out of date".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/trainer/SKILL.md

@@ -1,6 +1,8 @@
 ---
 name: trainer
 surfaces: [claude-code, library]
+summary: "Two-phase brain curation (score candidates, write past quality gate)"
+use_when: "Periodic brain pruning and curation"
 description: Two-phase brain curation — scan session logs to score candidate learnings (READ), then write the ones that pass the quality gate as generalized knowledge entries (WRITE). Use periodically to keep the brain useful, not noisy.
 ---