Add spec for enhanced notification categorization

[Copilot]

The current keyword-only notification categorization misclassifies chat responses and lacks channel/agent awareness. This PR documents a more robust categorization approach that prioritizes structured metadata and user-configurable rules.

• Spec overview
  • Defines layered categorization pipeline (chat toggle → structured metadata → user rules → keyword fallback).
• Data model & settings
  • Specifies notification metadata fields, settings additions, and migration defaults.
• UI & mapping guidance
  • Outlines minimal settings UI changes and channel/category mappings.

Example (proposed user rule format):

{
  "pattern": "invoice|receipt",
  "isRegex": true,
  "category": "email",
  "enabled": true
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Enhanced notification categorization system</issue_title>
<issue_description>## Current Behavior

Notifications are categorized by simple keyword matching in the message content:

• Health: blood sugar, glucose, cgm, mg/dl
• Urgent: urgent, critical, emergency
• Reminder: reminder
• Email: email, inbox, gmail
• Calendar: calendar, meeting, event
• Build: build, ci, deploy
• Stock: stock, in stock, available now
• Info: everything else (default)

Problem

This approach can be confusing - if Claude's response mentions 'email' while answering a general question, it gets categorized as an email notification.

Proposed Improvements

1. Folder/Channel-based categorization - categorize by which channel/session the message came from
2. Agent-based categorization - different notification settings per agent
3. User-defined rules - regex or keyword rules that users can customize
4. Smart categorization - use the gateway's intent/context if available
5. Separate chat toggle - simple on/off for all chat responses vs. system notifications

Notes

Current keyword-based system is documented in Settings UI with explanatory text.</issue_description>

<agent_instructions>Consider how you’d implement this. Make a spec </agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Enhanced notification categorization system #12

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