Remove duplicated HIGH-SIGNAL block in code-review, add a dup test

[waybarrios]

Summary

• The **Critical: only HIGH-SIGNAL issues should be flagged.** block appeared
  twice, back to back, in code-review's SKILL.md and command. The second copy
  was a strict subset of the first (it dropped the "Concrete edge cases" bullet),
  so it was pure redundancy. The duplicate is removed; the complete first copy stays.
• The skill-lint suite gains a per-skill "no duplicated block" check (no
  non-empty line ≥ 50 chars repeats within a SKILL.md or its command), catching
  this class of copy-paste error going forward.
• SKILL.md and command stay in sync.

Validation

• Deterministic: npm test → 55/55 pass (44 prior + 11 new duplicate-block
  checks; code-review: no duplicated block ✔).
• Behavioral: ran code-review locally via opencode run against a fixture
  with a planted off-by-one bug (return items[idx + 1]). Both models caught it at
  confidence 100 — full responses below.

Model outputs

DeepSeek V4 Flash Free — full finding

**Summary of change:** Modifies `util.py:get_item()` to add a comment and change the return from `items[idx]` to `items[idx + 1]`.

---

## Issues Found

### Critical

**1. Off-by-one bug in `get_item` — `util.py:3`**
- **Severity:** Critical
- **Confidence:** 100
- **Problem:** `return items[idx + 1]` returns the element *after* the requested index. The comment added in the same change confirms the intent was "return the item at idx" — the code contradicts it.
- **Category:** Logic error (off-by-one)
- **Reproduction scenario:** `get_item([10, 20, 30], 0)` returns `20` instead of `10`; `get_item([10, 20, 30], 2)` raises `IndexError`.
- **Fix:** Change `items[idx + 1]` to `items[idx]` on line 3.

One issue found and fixed. The change introduced a clear off-by-one bug on `util.py:3` — `items[idx + 1]` should be `items[idx]`. The comment added in the same diff confirms the intent was to return the item at `idx`, not `idx + 1`. Fix has been applied.

GPT-5.5 — full finding

**Important**
`util.py:3`
Severity: Important
Confidence: 100
Problem: `get_item(items, idx)` now returns `items[idx + 1]`, so it returns the next element instead of the requested index and fails at the last valid index.

Reproduction:
```python
get_item(["a", "b"], 0)  # returns "b", expected "a"
get_item(["a", "b"], 1)  # raises IndexError, expected "b"
```

Fix: restore `return items[idx]`.

Bug category: Logic error / off-by-one boundary error.

Both runs used opencode run --command-equivalent prompts on the same fixture; GPT-5.5 additionally dispatched the seven reviewer passes as parallel sub-agents.