docs(index): place experiment-spec in the relationships diagram

Missed in 83b069a. #15 asked for three things and the table row was only two
of them: experiment-spec also belongs in the Skill Relationships tree, parallel
to feature-spec and downstream of spec-driven-dev, since it is the research/ML
substitute for feature-spec rather than a separate lifecycle.

dream needs no edge, same as trainer and session-retrospective — the issue said
so and that part was right.

Refs #15
Bump-Type: patch

SKILLS_INDEX.md

@@ -109,14 +109,16 @@ problem-analysis
             │
             ├── spec-driven-dev (PM-kickoff lifecycle)
             │       │
-            │       └── feature-spec (per-feature, mid-implementation)
-            │               │
-            │               └── planning
-            │                       │
-            │                       └── atdd ──→ tdd (foundation)
-            │                                ├──→ debug (when tests fail)
-            │                                ├──→ clean-code (REFACTOR phase)
-            │                                └──→ test-design (review)
+            │       ├── feature-spec (per-feature, mid-implementation)
+            │       │       │
+            │       │       └── planning
+            │       │               │
+            │       │               └── atdd ──→ tdd (foundation)
+            │       │                        ├──→ debug (when tests fail)
+            │       │                        ├──→ clean-code (REFACTOR phase)
+            │       │                        └──→ test-design (review)
+            │       │
+            │       └── experiment-spec (research/ML phase, instead of feature-spec)
             │
             └── planning