ci: publish agentkey.skill asset on each release

[lxcong]

Summary

Restores the agentkey.skill release artifact that was produced by the retired scripts/release.sh but has been missing since v1.0.0.

The old release.sh zipped skills/agentkey/ (with SKILL.md at the zip root, no agentkey/ prefix) and attached it to the GitHub Release. This PR wires the same build into the release-please workflow so every automated release carries the asset.

What changed

.github/workflows/release-please.yml — now has two jobs:

1. release-please — unchanged behavior, now also exposes release_created and tag_name as outputs
2. publish-skill-asset — new, gated on release_created == 'true'. Builds the zip and uploads it via gh release upload --clobber

Artifact structure (verified byte-identical to v0.4.4)

SKILL.md              (zip root)
scripts/check-mcp.sh
scripts/check-update.sh

Excludes: *.DS_Store, __pycache__/*, *.pyc

Backfill note

v1.0.0 and v1.1.0 have no .skill asset because the workflow didn't exist yet. After this PR merges I can backfill them manually in one command per tag (no code change needed).

Test plan

• Local zip -r dry-run produces the expected 4-file archive, matches v0.4.4 layout
• YAML syntax valid (python3 -c "import yaml; yaml.safe_load(…)")
• commitlint passes on PR title
• Post-merge: no release cut yet (this PR is ci:), so no artifact. Will verify on next feat:/fix: release.
• v1.0.0 and v1.1.0 backfilled manually after merge