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"SKILLS_INDEX.md is out of date".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

bin/check-index

@@ -1,42 +0,0 @@
-#!/bin/sh
-# check-index — asserts SKILLS_INDEX.md lists exactly the skills that exist.
-#
-# The index is hand-maintained and drifts silently in both directions: a skill
-# ships with no row (`dream` and `experiment-spec` did — their rows lived on
-# branches squash-merged after a directory reorg, so the SKILL.md landed and
-# the row did not, #15), or a row outlives the skill it names. Neither errors
-# anywhere; the library just misreports itself to every harness that reads it.
-#
-# Usage: check-index [checkout-root]   (defaults to this script's repo root)
-
-set -u
-
-ROOT=${1:-$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)}
-INDEX="$ROOT/SKILLS_INDEX.md"
-fail=0
-
-if [ ! -f "$INDEX" ]; then
-  echo "ERROR: missing SKILLS_INDEX.md" >&2
-  exit 1
-fi
-
-# A row is a table line whose first cell is the skill name in backticks.
-listed=$(sed -n 's/^|[[:space:]]*`\([^`]*\)`.*/\1/p' "$INDEX" | sort -u)
-
-for dir in "$ROOT"/skills/*/; do
-  name=$(basename "$dir")
-  [ -f "$dir/SKILL.md" ] || continue
-  printf '%s\n' "$listed" | grep -qx "$name" || {
-    echo "ERROR: skills/$name has no row in SKILLS_INDEX.md" >&2
-    fail=1
-  }
-done
-
-for name in $listed; do
-  [ -f "$ROOT/skills/$name/SKILL.md" ] || {
-    echo "ERROR: SKILLS_INDEX.md lists '$name', which has no skills/$name/SKILL.md" >&2
-    fail=1
-  }
-done
-
-exit $fail

bin/generate-index

@@ -0,0 +1,85 @@
+#!/bin/sh
+# generate-index — rebuild SKILLS_INDEX.md's table from skill frontmatter.
+#
+# The index was hand-maintained and drifted: `dream` and `experiment-spec`
+# shipped with no row at all, because their rows lived on branches that were
+# squash-merged after a directory reorg (#15). A gate catches that after the
+# fact; generating removes the class (#29).
+#
+# The curated summary and trigger live in each SKILL.md as `summary:` and
+# `use_when:` rather than being derived from `description:` — description is a
+# paragraph written for model selection, and truncating it produces a worse
+# table than the one being replaced.
+#
+# Everything outside the markers is left untouched: the wiring docs, the
+# relationships tree and the trigger table are prose, not data.
+#
+#   generate-index [root]           rewrite the table in place
+#   generate-index --check [root]   exit 1 if the table is out of date
+
+set -u
+
+CHECK=0
+if [ "${1:-}" = "--check" ]; then CHECK=1; shift; fi
+ROOT=${1:-$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)}
+INDEX="$ROOT/SKILLS_INDEX.md"
+
+BEGIN='<!-- BEGIN skills-table -->'
+END='<!-- END skills-table -->'
+
+[ -f "$INDEX" ] || { echo "ERROR: missing $INDEX" >&2; exit 1; }
+grep -qF "$BEGIN" "$INDEX" || { echo "ERROR: $INDEX has no $BEGIN marker" >&2; exit 1; }
+grep -qF "$END" "$INDEX" || { echo "ERROR: $INDEX has no $END marker" >&2; exit 1; }
+
+# field <file> <key> — a scalar frontmatter value, quotes stripped.
+field() {
+  sed -n "s/^$2:[[:space:]]*//p" "$1" | head -1 | sed 's/^"//; s/"$//'
+}
+
+table=$(mktemp)
+fail=0
+
+{
+  printf '| Skill | Description | Use when | Surfaces |\n'
+  printf '|-------|-------------|----------|----------|\n'
+} > "$table"
+
+for dir in "$ROOT"/skills/*/; do
+  [ -f "$dir/SKILL.md" ] || continue
+  name=$(basename "$dir")
+  summary=$(field "$dir/SKILL.md" summary)
+  use_when=$(field "$dir/SKILL.md" use_when)
+  surfaces=$(field "$dir/SKILL.md" surfaces | tr -d '[]')
+
+  if [ -z "$summary" ] || [ -z "$use_when" ]; then
+    echo "ERROR: skills/$name has no 'summary:'/'use_when:' — the index table is generated from them" >&2
+    fail=1
+    continue
+  fi
+  printf '| `%s` | %s | %s | %s |\n' "$name" "$summary" "$use_when" "$surfaces" >> "$table"
+done
+
+if [ "$fail" -ne 0 ]; then
+  rm -f "$table"
+  exit 1
+fi
+
+out=$(mktemp)
+awk -v begin="$BEGIN" -v end="$END" -v tablefile="$table" '
+  $0 == begin { print; while ((getline line < tablefile) > 0) print line; skip = 1; next }
+  $0 == end   { print; skip = 0; next }
+  !skip       { print }
+' "$INDEX" > "$out"
+rm -f "$table"
+
+if [ "$CHECK" -eq 1 ]; then
+  if cmp -s "$out" "$INDEX"; then
+    rm -f "$out"
+    exit 0
+  fi
+  rm -f "$out"
+  echo "ERROR: SKILLS_INDEX.md is out of date — run 'task index' and commit the result" >&2
+  exit 1
+fi
+
+mv "$out" "$INDEX"