chore(ci): remove external skill publishing

[altaywtf]

An obsolete external skill registry was restored after its shared action had been deleted.

• Fix: remove its publication path and keep the attach CLI skill as repository-local source only.
• Live state: the skill-release environment and its token are deleted.

[slopzapper]

✨ CLEAN

The PR deletes the Tessl publish workflow, smoke script, and plugin manifest, and updates the two docs that described that path. No leftover callers or Tessl publish references remain.

[Copilot]

Pull request overview

Removes the obsolete CI/publication path for the skills/attach-cli skill so it remains a repository-local skill source, aligning the repo with the current “live state” where the skill-release environment/token no longer exist.

Changes:

• Delete the GitHub Actions workflow that published the skill externally.
• Remove the Tessl plugin manifest and the publish-install smoke test script used for external verification.
• Update release/agent documentation to remove references to external skill publishing.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
skills/attach-cli/.tessl-plugin/plugin.json	Removed external skill registry manifest metadata (name/version/skills) used for publishing.
scripts/verify-published-skill.sh	Removed the external “published skill installability + Codex discovery” smoke test script.
docs/releasing.md	Removed the “Tessl skill” release section so releasing docs reflect current publication paths.
AGENTS.md	Updated orientation references to describe the attach-cli skill as a repo-local source (and adjusted the release entry accordingly).
.github/workflows/publish-skill.yml	Removed the CI workflow that published the skill via the deleted skill-release environment.

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