chore(skill): bump emcli skill version to 0.6.0 (mirror binary)

Mirror the skill metadata version to the binary release and document the
three skill-package touch points in RELEASING.md so they bump together.

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

skills/emcli/SKILL.md

@@ -4,7 +4,7 @@ description: Read and send email through emcli, a guard-railed CLI gateway that
 compatibility: Requires the emcli binary (run scripts/install.sh to fetch it; needs curl or wget and a Linux/macOS/Windows shell) and the EMCLI_KEY environment variable, which the orchestrator provides. Needs network access to the configured mail server.
 metadata:
   author: steve
-  version: "0.4.0"
+  version: "0.6.0"
   homepage: "https://gitea.dcglab.co.uk/steve/emcli"
 ---