docs: add user manual; expand README

USER-MANUAL.md — full user-facing guide: setup (EMCLI_KEY/EMCLI_DB), adding
accounts (incl. Gmail app-password walkthrough), admin commands (account /
whitelist / config / audit / doctor / init), agent commands (list/get/search/
ack/send) with exact flags, the JSON envelope + error codes, enforcement rules,
troubleshooting, and a cheat sheet. README now summarizes emcli and links it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -1 +1,20 @@
 # emcli
+
+A single command-line program that mediates all email access for an AI agent. The agent never
+holds your email password and never talks to the mail server directly — every read and send goes
+through `emcli`, which enforces the limits you configure (read-only/read-write, sender and
+recipient whitelists, subject filters). Even with faulty instructions, the agent can't read mail
+it isn't permitted to see or send mail to people it isn't permitted to contact.
+
+## Getting started
+
+```bash
+export EMCLI_KEY="$(head -c 32 /dev/urandom | base64)"   # one-time: generate & save a key
+emcli init                                                # create the DB, add your first account
+emcli doctor                                              # confirm it connects and authenticates
+```
+
+## Documentation
+
+See the **[User Manual](USER-MANUAL.md)** for full setup, account configuration (including Gmail
+app passwords), the agent and admin command reference, the JSON output format, and troubleshooting.