feat(search): add from/date filters and require explicit search criteria

[realugbun]

Description

Two improvements to search_emails:

1. New filters — from, since, before, sent_since, sent_before.
   The underlying ImapFlow SearchObject already supported all of these;
   only the plumbing was missing. Mirrors the existing to field and the
   equivalent filters already exposed on list_emails.

2. Reject all-empty calls — previously, search_emails with no query
   and no filters ran client.search({}, { uid: true }), which matches
   every UID in the mailbox. This made the tool indistinguishable from
   list_emails for empty input and silently masked bugs where unknown
   parameter names were stripped by the Zod schema (e.g. a caller passing
   keyword: instead of query: would see a full-mailbox response with
   no hint that their argument had been dropped). Now throws a clear error
   pointing callers to list_emails.

Motivating example

A common ask is "emails from sender X in the last N days." That used to
require list_emails (which already had from + since), even though
search_emails was the more semantically obvious tool for filtered
lookups. After this PR a single call works:

{
  "account": "work",
  "from": "billing@example.com",
  "since": "2026-05-01T00:00:00Z"
}

Or combined with a keyword:

{
  "account": "work",
  "query": "invoice",
  "since": "2026-04-01T00:00:00Z",
  "has_attachment": true
}

Type of change

• New feature (non-breaking change that adds functionality)
• Breaking change — search_emails with no filters now errors
• Bug fix
• Documentation update
• Refactor

Checklist

• Code follows project style (Biome + ESLint — pnpm check passes)
• pnpm check passes
• pnpm typecheck passes
• All unit tests pass (pnpm test) — 155/155, including 5 new
  searchEmails criteria-builder tests
• Tool description in emails.tool.ts updated to reflect new params

Notes for reviewer

• The breaking-change behavior could alternatively be a deprecation
  warning that returns the full mailbox for one release. Happy to soften
  if you prefer a gentler landing.
• Pagination math is unchanged.
• Resolves: Add date-based filtering to search (before/after/range) #31