feat: GmailLabelManager

[lexi-chief-of-staff]

Add label CRUD to GmailDriver: create/list/update/delete labels + message labeling helpers.

Spec:

• createLabel(name, color, ?parent) → label ID
• listLabels() → paginated labels
• deleteLabel(id)
• applyLabel(msgIds, labelId) → batchModify wrapper
• Tests: full CRUD cycle

Powers Lexi rules like auto-label 'pay:bill', 'archive:promo'. Refs batchModify.