feat: add create_group and leave_group MCP tools

[jayeshkaithwas]

Summary

Adds two new MCP tools so the agent can manage groups, not just read/send into existing ones.

• create_group(name, participants, is_community=False, community_parent_jid="") — wraps whatsmeow.CreateGroup. Participants accept either bare phone numbers (country code, no +) or full JIDs. The caller's own JID is added implicitly by WhatsApp, so it must not be included. Optionally creates a community parent (is_community=True) or attaches the new group as a sub-group of an existing community (community_parent_jid). Persists the resulting chat to the local SQLite so it shows up in list_chats right away.
• leave_group(jid) — wraps whatsmeow.LeaveGroup. WhatsApp has no "delete group" primitive — this is the closest action; other members continue to see the group with you marked as left.

Changes

• whatsapp-bridge/main.go: new /api/create_group and /api/leave_group endpoints + their request/response types and handlers.
• whatsapp-mcp-server/whatsapp.py: matching create_group / leave_group HTTP helpers.
• whatsapp-mcp-server/main.py: tool registrations.
• README.md: tool list updated with the two new entries.

No dependency bumps. No changes to existing tools or storage schema. Builds cleanly against the whatsmeow version pinned in go.mod.

Test plan

• go build clean against the existing pinned whatsmeow.
• python3 -m py_compile clean on both server files.
• Manually exercised on a live bridge: created a 2-person group via POST /api/create_group, confirmed it appeared in WhatsApp on both ends and in list_chats. Then left it via POST /api/leave_group and confirmed the "left" event on the other device.
• Reviewer to verify behaviour against their own paired account.

Notes / open questions

• I deliberately kept the surface minimal (just name, participants, plus the two community knobs that whatsmeow.ReqCreateGroup already exposes here). Happy to add announce/locked/ephemeral/join_approval_required in a follow-up — they need a newer whatsmeow than the one currently pinned.
• create_group writes the new chat row directly to the bridge SQLite. WhatsApp's own events.JoinedGroup would also do this asynchronously; doing it eagerly just makes the tool's response immediately visible to subsequent list_chats calls in the same MCP session. Open to dropping that line if you'd rather rely on the event path alone.
• Group names are silently capped by WhatsApp at 25 chars — I validate this client-side in the bridge and return a clear error rather than letting whatsmeow surface a 406.

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