feat(skills): add pdf-reader skill (#772)

Thanks @glifocat! Clean skill package — good docs, solid tests, nice intent files. Pushed a small fix for path traversal on the PDF filename before merging.

.claude/skills/add-pdf-reader/modify/src/channels/whatsapp.test.ts.intent.md

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+# Intent: src/channels/whatsapp.test.ts modifications
+
+## What changed
+Added mocks for downloadMediaMessage and normalizeMessageContent, and test cases for PDF attachment handling.
+
+## Key sections
+
+### Mocks (top of file)
+- Modified: config mock to export `GROUPS_DIR: '/tmp/test-groups'`
+- Modified: `fs` mock to include `writeFileSync` as vi.fn()
+- Modified: Baileys mock to export `downloadMediaMessage`, `normalizeMessageContent`
+- Modified: fake socket factory to include `updateMediaMessage`
+
+### Test cases (inside "message handling" describe block)
+- "downloads and injects PDF attachment path" — verifies PDF download, save, and content replacement
+- "handles PDF download failure gracefully" — verifies error handling (message skipped since content remains empty)
+
+## Invariants (must-keep)
+- All existing test cases unchanged
+- All existing mocks unchanged (only additive changes)
+- All existing test helpers unchanged
+- All describe blocks preserved