docs(index): add dream + experiment-spec rows, and gate the index

Both skills shipped without an index row. The rows existed — on the feature
branches deleted in #22 — and were dropped by the squash-merge that followed
a directory reorg: the SKILL.md landed under skills/, the row did not. Nothing
errored, the index just under-reported the library to every harness reading it.

Adding two rows by hand fixes today and not tomorrow, so the assertion goes in
`task check` via bin/check-index, following the same pattern as
bin/should-skip-release: a real script with a real test, not untested logic
buried in a Taskfile. It checks both directions — a skill with no row, and a
row naming a skill that no longer exists.

Verified by mutation: deleting the `dream` row turns `task check` red and the
report names it.

Closes #15
Bump-Type: patch

SKILLS_INDEX.md

@@ -19,9 +19,11 @@ This index lists all available engineering skills. Load the full SKILL.md on dem
 | `gitea-ci` | Gitea CI/CD pipeline with act_runner, buildah, k3s, GitHub mirror | Setting up or debugging CI on this infra |
 | `debug` | Hypothesis-first debugging discipline | Any test failure or bug whose cause is not immediately obvious |
 | `feature-spec` | Implementation-level feature spec (tighter than spec-driven-dev) | Scoping one feature inside an already-specced project |
+| `experiment-spec` | Rigorous experiment spec — falsifiable hypothesis, quantitative acceptance, null-result protocol | Scoping a research/ML phase instead of a feature |
 | `playwright` | Playwright E2E testing — setup, HTMX patterns, SSE streams, CI wiring | Adding browser tests to any project with a web UI |
 | `session-retrospective` | Surface non-obvious learnings from a session log | After a coding session ends, before context is lost |
 | `trainer` | Two-phase brain curation (score candidates, write past quality gate) | Periodic brain pruning and curation |
+| `dream` | Reflective consolidation pass over an agent's memory directory | Pruning/merging memory files, rebuilding a memory index |
 | `grill-me` | Structured plan interrogation — Quick Poke, Full Grill, Pre-mortem | Stress-testing a plan before committing; end of Diamond 1; before promoting to pre-prod |
 | `own-the-task` | Delegation ownership — Intake → Brief → Plan → Execute | A task is handed over open-ended; "take this off my plate"; before a plan exists |
 | `telos-load` | Load TELOS intention substrate at session start | Starting any koala session; before architectural decisions; CAD pipeline entry |