feat(skill): add emcli Agent Skill (agentskills.io standard)

skills/emcli/ — an Agent Skill teaching an agent to read and send mail through
emcli's JSON agent commands:

- SKILL.md: name/description (what + when + trigger keywords), compatibility,
  metadata; body covers the security model (agent-only commands, never touch
  EMCLI_KEY), setup, the list→get→ack workflow, sending, and enforcement
  awareness. Frontmatter validated against the spec (name matches dir; desc
  574/1024; compatibility 239/500); body 146 lines (<500).
- scripts/install.sh: detects OS/arch, downloads the release binary, verifies
  the sha256 checksum when present, fails gracefully. Release tag/assets
  (v0.4.0, emcli_<ver>_<os>_<arch>) are placeholders until the first release.
- references/{commands.md,install.md}: full agent command reference (flags, JSON
  shapes, error codes, enforcement) and install options, loaded on demand.

README links to the skill.

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

README.md

@@ -18,3 +18,7 @@ emcli doctor                                              # confirm it connects
 
 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.
+
+For AI agents, **[`skills/emcli`](skills/emcli/SKILL.md)** is an
+[Agent Skill](https://agentskills.io) that teaches an agent to read and send mail through `emcli`,
+including a binary installer.