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"SKILLS_INDEX.md is out of date".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

Taskfile.yml

@@ -61,8 +61,13 @@ tasks:
         done
         exit $fail
 
+  index:
+    desc: 'Regenerate SKILLS_INDEX.md''s table from skill frontmatter.'
+    cmds:
+      - "'{{.TASKFILE_DIR}}/bin/generate-index' '{{.TASKFILE_DIR}}'"
+
   check:
-    desc: 'Structure gate — layout, frontmatter, manifest/tag parity. Runs in CI.'
+    desc: 'Structure gate — layout, frontmatter, surfaces, generated index, manifest/tag parity. Runs in CI.'
     cmds:
       - |
         fail=0
@@ -94,10 +99,11 @@ tasks:
           count=$((count + 1))
         done
 
-        # 3. SKILLS_INDEX.md lists exactly the skills that exist. Delegated
-        #    to bin/check-index so the rule is covered by
-        #    tests/check_index_test.sh rather than living untested here.
-        sh '{{.TASKFILE_DIR}}/bin/check-index' '{{.TASKFILE_DIR}}' || fail=1
+        # 3. SKILLS_INDEX.md's table is generated from frontmatter, so the
+        #    assertion is that regenerating it changes nothing. This replaces
+        #    bin/check-index: a generated table cannot drift in either
+        #    direction, so there is no separate rule left to check.
+        sh '{{.TASKFILE_DIR}}/bin/generate-index' --check '{{.TASKFILE_DIR}}' || fail=1
 
         # 4. Every skill says which surfaces it ships to, and names only
         #    surfaces that exist — a wrong value silently drops a skill from a