feat(checks): add duplicate-name rule

[protosphinx]

Why

Two skills that declare the same name: in frontmatter cause ambiguous
resolution at runtime. The model must pick one and the other silently
never fires. This is a real authoring bug with no visible signal in
model output - exactly the class of bug skillcheck exists to catch.
Addresses the v1.0 goal of catching every authoring pitfall before CI.

What

• Adds checkDuplicateNames(skills) in src/checks.ts, a cross-skill
  check that groups validated skills by lowercased name and emits one
  duplicate-name warn per skill in any group with two or more members.
  Each diagnostic message names the conflicting file so the author knows
  exactly which skill to rename.
• Calls checkDuplicateNames from runChecks alongside checkCollisions.
• Adds the rule to the README rule table.

Tests

Four new cases in test/checks.test.ts:

• Two skills with the same name produce exactly two duplicate-name warns.
• Skills with different names produce no duplicate-name diagnostic.
• Three skills sharing a name produce exactly three warns (one per skill).
• The diagnostic message for a skill names the other file in the conflict.

Self-merge gate

• all CI checks pass
• LOC delta < 250 (added + removed)
• no public-API surface change
• no runtime-dependency additions
• no workflow file changes
• tests added or extended

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