fix(slide-builder): brand kit PUT is destructive, not a partial update

[charliebirch-cmyk]

Summary

The "Update Brand Kit" section in relevance-slide-builder/SKILL.md claims "Partial update — only provided fields are changed". That contradicts the destructive-semantics warning already documented in the SE Agent Builder downstream:

• .claude/rules/universal.md "Brand Kits" section: PUT wipes any array field (colors, logos, inspiration_photos) that is omitted from the body; scalar fields (name, brand_tone, voice, fonts) are preserved. Fix pattern is fetch-merge-save.
• build-kit/tools/platform-tool-gotchas.md "Brand Kit API": same warning, plus the singular/plural endpoint naming mismatch (GET /branding_kit/{id} vs PUT|DELETE /branding_kits/{id}).

This PR brings the cc-plugin skill docs into alignment so every project consuming this plugin gets the same guidance — not just the SE Agent Builder repo.

Changes

• Rewrite the "Update Brand Kit" example as fetch-merge-save, with a BAD counter-example showing the partial PUT that wipes arrays.
• Annotate the Brand Kit Endpoints table with the singular/plural naming inconsistency so future readers don't "fix" it.

Docs-only; no API contract change.

Test plan

• Render the SKILL.md locally — headings, code fences, table all render correctly
• Reviewer sanity-check that the fetch-merge-save pattern matches how the rest of the plugin documents destructive PUTs (compare to relevance_save_agent_draft guidance)

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