feat(skills): add whatsapp-formatting container skill

Teaches agents WhatsApp's mention syntax (@<phone-digits>, never display
names) and where to find the sender's phone JID in inbound metadata
(content.sender). Without this, agents default to @<displayName>, which
WhatsApp can't tag — it just renders as plain text with no notification.

Two files:

- SKILL.md — frontmatter + description so the Claude Agent SDK can
  discover it via skill metadata for ad-hoc lookups.
- instructions.md — always-on guidance. claude-md-compose.ts inlines
  any skill that ships an instructions.md into every group's CLAUDE.md
  on container spawn, so the rule is in the agent's context for every
  reply (not just when the agent decides to invoke the Skill tool).

Mirrors the existing container/skills/slack-formatting/ layout for the
analogous Slack mrkdwn rules. Pairs with the adapter-side fix on the
`channels` branch that wires `mentions` through to Baileys' contextInfo
— both layers are needed for tags to render end-to-end.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/skills/whatsapp-formatting/SKILL.md

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+---
+name: whatsapp-formatting
+description: Format messages for WhatsApp, including mentions that render as real WhatsApp tags. Use when responding in a WhatsApp conversation (platform_id / chatJid ends with @s.whatsapp.net or @g.us).
+---
+
+# WhatsApp Message Formatting
+
+WhatsApp uses its own lightweight markup and a phone-number-based mention syntax. The host's WhatsApp adapter (Baileys) handles markdown conversion automatically, but **mentions are only protocol-level mentions if you use the right syntax** — otherwise they render as plain text and don't notify the recipient.
+
+## How to detect WhatsApp context
+
+You're in a WhatsApp conversation when any of these are true:
+- The chat JID / platform id ends with `@s.whatsapp.net` (1-on-1 DM)
+- The chat JID / platform id ends with `@g.us` (group)
+- Your inbound message metadata has `chatJid` matching the above
+
+## Mentions — the important part
+
+To tag a user so their name appears **bold and clickable** in WhatsApp and they get a push notification, write the `@` followed by their phone number digits (no `+`, no spaces, no display name):
+
+```
+@15551234567 can you confirm?
+```
+
+The adapter scans your outgoing text for `@<digits>` (5–15 digits, optional leading `+` is stripped) and tells WhatsApp to render them as real mention tags.
+
+**The sender's phone JID is always in your inbound message metadata.** When a user writes to you, inbound `content.sender` looks like `15551234567@s.whatsapp.net`. The part before the `@` is exactly what you put after `@` when tagging them back.
+
+### Wrong vs right
+
+| You write | What recipients see |
+|-----------|---------------------|
+| `@Adam can you...` | Plain text `@Adam`. No tag, no notification. |
+| `@15551234567 can you...` | Bold/blue **@Adam** (or whatever name they're saved as), notification fires. |
+| `@+15551234567 ...` | Same as above — adapter strips the `+`. |
+
+### Picking who to tag
+
+- In a DM, there's no real need to tag the recipient (they already see every message), but tagging still works if you want emphasis.
+- In a group, look at the `participants` / inbound `content.sender` to find the JID of the person you mean. Don't guess from display names — pushNames can collide and are not reliable.
+- If you don't know the JID, just refer to the person by name in plain prose. Don't write `@<name>` — it won't tag and it will look like a tag that failed.
+
+## Text styles
+
+WhatsApp uses single-character delimiters, *not* doubled like standard Markdown.
+
+| Style | Syntax | Renders as |
+|-------|--------|------------|
+| Bold | `*bold*` | **bold** |
+| Italic | `_italic_` | *italic* |
+| Strikethrough | `~strike~` | ~strike~ |
+| Monospace | `` `code` `` | `code` |
+| Block monospace | ```` ```block``` ```` | preformatted block |
+
+The adapter converts standard Markdown (`**bold**`, `[link](url)`, `# heading`) to the WhatsApp-native form automatically, so you don't have to think about it — but be aware that single asterisks become italics, not bold.
+
+## What not to do
+
+- Don't write `<@U123>` (that's Slack), `<@!123>` (Discord), or any other channel's mention syntax.
+- Don't paste a full JID like `@15551234567@s.whatsapp.net` in the text — only the digits before the JID's `@` go after your `@`.
+- Don't try to tag display names. WhatsApp has no display-name-based mention API.

container/skills/whatsapp-formatting/instructions.md

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+## WhatsApp mentions — always use phone digits
+
+When you are replying in a WhatsApp conversation (the inbound message's `chatJid` ends with `@s.whatsapp.net` for a DM or `@g.us` for a group), and you want to tag a person so their name appears **bold and clickable** with a push notification, write `@` followed by their phone-number digits — never the display name.
+
+**The sender's phone JID is in your inbound message metadata** at `content.sender` (e.g. `15551234567@s.whatsapp.net`). The part before the `@` is exactly what you put after `@` when tagging them.
+
+| You write | What recipients see |
+|-----------|---------------------|
+| `@Adam, can you...` | Plain text. No tag, no notification. |
+| `@15551234567, can you...` | Bold/blue **@Adam** (whatever name they're saved as), notification fires. |
+| `@+15551234567 ...` | Same as above — the adapter strips the `+` automatically. |
+
+The host adapter scans your outbound text for `@<5–15 digits>` (with optional leading `+`) and tells WhatsApp to render those as real mention tags. If the digits aren't in the text, the tag doesn't render — no exceptions.
+
+### In groups
+
+Tag the person you're addressing using their JID from inbound metadata (look at the most recent message from them). Don't guess — pushNames collide and aren't reliable.
+
+If you don't know someone's JID, refer to them by name in plain prose. Do not write `@<displayname>` hoping it works.