docs(skill): split setup into AGENTIC-MANUAL.md; keep SKILL.md lean

The SKILL.md body loads into context on every activation, so one-time install/
setup prose was wasted context once emcli is running. Move it out:

- New AGENTIC-MANUAL.md: get-the-files bootstrap, binary install (incl. options
  and build-from-source, folding in the old references/install.md), EMCLI_KEY,
  account discovery. Fetched only during first-time setup.
- SKILL.md trimmed (182→~145 lines) to the recurring path: security model, a short
  "Files & first run" pointer + per-session preflight, the list/get/ack/send
  workflow, JSON envelope, command table, enforcement, do/don't.
- Remove references/install.md (folded in); fix RELEASING.md pointer.

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

RELEASING.md

@@ -49,8 +49,8 @@ published. If it ever doesn't fit your runner setup, fall back to Option A.
 ## After a release
 
 The skill installer defaults to `EMCLI_VERSION=v0.4.0`. When you cut a different version, either
-publish under that tag or update the default in `skills/emcli/scripts/install.sh` (and the note in
-`skills/emcli/references/install.md`).
+publish under that tag or update the default in `skills/emcli/scripts/install.sh` (and the options
+table in `skills/emcli/AGENTIC-MANUAL.md`).
 
 ## Versioning