chore: enforce mandatory PR review phase in /pr skill (#901)

## Summary

Adds an explicit **DO NOT SKIP** directive to Phase 2 (Automated PR
review) of the `/pr` skill, preventing the review subagent phase from
being bypassed.

- Adds mandatory enforcement language at the top of Phase 2
- The only exception remains docs-only changes (already documented)

During the libp2p v3 upgrade (#900), the review phase was skipped
because the changes appeared "mechanical". The review — when eventually
run — caught a `find()` vs `findLast()` bug in the multiaddr migration
that would have broken circuit relay address routing. This change
ensures that doesn't happen again.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk documentation-only change that tightens process guidance
without affecting runtime code. Main risk is minor workflow friction if
the instruction is misinterpreted.
> 
> **Overview**
> Adds an explicit **“MANDATORY — DO NOT SKIP”** directive to Phase 2 of
the `/pr` skill (`.claude/skills/pr/SKILL.md`), requiring automated
subagent review before PR creation.
> 
> Keeps the existing exception for docs-only diffs, but clarifies that
all other change types must run the review phase even when changes
appear “mechanical” or “well-tested.”
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
a7ad4df. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sirtimid

.claude/skills/pr/SKILL.md

@@ -30,6 +30,8 @@ When asked to create a pull request, follow these steps:
 
 ## Phase 2: Automated PR review (parallel subagents)
 
+**MANDATORY — DO NOT SKIP.** This phase must run before any PR is created, regardless of how "simple", "mechanical", or "well-tested" the changes appear. Subtle bugs (e.g., semantic mismatches in API migrations) hide in exactly the changes that seem safe to skip. The only exception is docs-only changes as described below.
+
 Before creating the PR, analyze the diff to decide **which review subagents to launch**. Not every PR needs every reviewer.
 
 ### Triage: classify the change