Unify message-list output shape across listy MCP tools #76

Problem

Each listy MCP tool returns messages in a slightly different shape:

Field sets per message row also diverge: `thread` drops `message_type` and `forwarded_from_name`, `digest` drops `reply_to_message_id` and `message_type`.

An AI agent that learned the shape of one tool trips on another. It also makes pagination resumption awkward — the cursor key names differ, and tools like `chats`/`contacts` don't expose pagination in the MCP schema at all despite supporting it server-side.

Proposal

Canonical message-list response:
```ts
{
messages: MessageRow[], // canonical array key
page: {
has_more: boolean,
next_cursor: string | null, // opaque cursor, hides before_id/after_id/sent_at mechanics
total?: number // optional — expensive count, opt-in via include_total param
}
}
```

Canonical `MessageRow` shape: every tool returns the same fields (with optional `fields: 'minimal' | 'full'` param for payload control). Default to `full`.

Apply to: `search`, `history`, `thread`, `digest` (shape per chat), and any future listy tools.

Also: expose pagination on `chats` and `contacts`/`address_book` where it's silently capped today (1,648 chats → default `limit: 50` truncates without `has_more` signal).

Migration strategy

Option A (clean break): bump MCP server version, breaking change. One-time pain.
Option B (opt-in): accept `format: 'v1' | 'v2'` param per tool, default v1 for now, switch default to v2 after clients update.

Prefer A since the consumer base is small (primarily the issue author's own Claude Code installs). Call out in SPEC / README.

Acceptance

• All listy MCP tools return `{ messages, page: { has_more, next_cursor, total? } }`
• Cursor is opaque (AI doesn't assemble `before_id + before_sent_at` manually)
• `has_more` always present so AI knows when to stop paginating
• `chats` and `address_book` paginate properly
• `include_total` param on `search` (default true on first page, false on follow-ups)
• Server instructions and tool descriptions updated with new shape