feat(cli): folders command, search --all-folders, empty-search hint

Fixes from testing email search (docs/enhancements-2026-07-07.md):

- New `folders` agent command lists the account's mailboxes (name,
  delimiter, selectable), INBOX first, so agents can discover archived
  mail outside INBOX.
- `search --all-folders` sweeps every selectable mailbox; each hit
  carries a `folder` field, `skipped_folders` reports mailboxes the
  server refused, and --limit caps visible results across the sweep.
  The sweep deliberately skips EnsureFolderBaseline so a read-only
  search never mutates list --new state.
- Empty search results include a generic `data.hint` with next steps.
  The hint is a fixed constant per mode, so the invisibility invariant
  holds: absent and policy-filtered mail produce byte-identical
  envelopes (codified in TestSearchEmptyHintIndistinguishableFromFiltered).
- Skill and user docs: document `--text` full-text search as
  best-effort (server-dependent); recommend --subject-contains/--from.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/enhancements-2026-07-07.md

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+Fixes identified from testing emcli email search (Morphy Richards purchase investigation).
+
+1. No folder discovery command
+
+emcli has no folders command, so the agent cannot enumerate available mailboxes. Searches default to INBOX only and miss archived mail (e.g. INBOX.Archive).
+
+Fix: Add a folders agent command to list available mailboxes.
+
+2. INBOX-only default too narrow
+
+When searching for a specific email, the agent should ideally search across all folders, but cannot without knowing folder names.
+
+Fix: Add an --all-folders flag to search to sweep all mailboxes at once.
+
+3. --text full-text search unreliable on some IMAP servers
+
+On myfamilyemail.co.uk, --text returned zero results for emails that --subject-contains found fine. The IMAP server either does not support full-text search or has an incomplete index.
+
+Fix: Document this limitation in the skill; recommend --subject-contains as the primary search method and treat --text as best-effort.
+
+4. No way to distinguish filtered vs missing vs wrong-folder
+
+All three cases return empty results with no error, making it impossible for the agent to know whether mail is absent, hidden by a whitelist, or simply in another folder.
+
+Fix: emcli (or the skill) should surface a diagnostic hint when results are empty — e.g. suggest running folders or retrying with --all-folders.