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.ts.intent.md

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+# Intent: src/channels/whatsapp.ts modifications
+
+## What changed
+Added PDF attachment download and path injection. When a WhatsApp message contains a PDF document, it is downloaded to the group's attachments/ directory and the message content is replaced with the file path and a usage hint. Also uses `normalizeMessageContent()` from Baileys to unwrap container types before reading fields.
+
+## Key sections
+
+### Imports (top of file)
+- Added: `downloadMediaMessage` from `@whiskeysockets/baileys`
+- Added: `normalizeMessageContent` from `@whiskeysockets/baileys`
+- Added: `GROUPS_DIR` from `../config.js`
+
+### messages.upsert handler (inside connectInternal)
+- Added: `normalizeMessageContent(msg.message)` call to unwrap container types
+- Changed: `let content` to allow reassignment for PDF messages
+- Added: Check for `normalized.documentMessage?.mimetype === 'application/pdf'`
+- Added: Download PDF via `downloadMediaMessage`, save to `groups/{folder}/attachments/`
+- Added: Replace content with `[PDF: attachments/{filename} ({size}KB)]` and usage hint
+- Note: PDF check is placed BEFORE the `if (!content) continue;` guard so PDF-only messages are not skipped
+
+## Invariants (must-keep)
+- All existing message handling (conversation, extendedTextMessage, imageMessage, videoMessage)
+- Connection lifecycle (connect, reconnect with exponential backoff, disconnect)
+- LID translation logic unchanged
+- Outgoing message queue unchanged
+- Group metadata sync unchanged
+- sendMessage prefix logic unchanged
+- setTyping, ownsJid, isConnected — all unchanged
+- Local timestamp format (no Z suffix)