chore(pstack): bump to 0.15.20

[poteto]

Why

This is a patch release of pstack. It ships the pstack changes that merged after the last version bump (#571, 0.15.18 → 0.15.19). This PR changes only the version. It does not change behavior.

Scope

• pstack/.cursor-plugin/plugin.json. Version 0.15.19 → 0.15.20.
• pstack/plugin.json (the Agent Plugins v1 root manifest from feat(pstack): add Agent Plugins v1 root manifest #576). Version 0.15.19 → 0.15.20. The two manifests must carry the same version. Cursor reads .cursor-plugin/plugin.json first, and Codex reads the root plugin.json first.

No other file in the repo holds the pstack version. The pstack entry in .cursor-plugin/marketplace.json has no version field, .claude-plugin/marketplace.json does not list pstack, and pstack has no changelog.

Included PRs

These pstack PRs merged after #571, newest first:

• fix(pstack): create-verification-skill readiness means the app is usable #573 fix(pstack): create-verification-skill readiness means the app is usable
• feat(pstack): add Agent Plugins v1 root manifest #576 feat(pstack): add Agent Plugins v1 root manifest
• fix(pstack): make the /poteto-mode chat the runner, poteto-agent a delegate #575 fix(pstack): make the /poteto-mode chat the runner, poteto-agent a delegate
• fix(pstack): worktree-audit checks MERGED against the default branch, not origin/main #574 fix(pstack): worktree-audit checks MERGED against the default branch, not origin/main
• fix(pstack): drop "Must always apply." from unslop description #572 fix(pstack): drop "Must always apply." from unslop description
• fix(watch-pr): paginate review threads instead of reading only the first 100 #484 fix(watch-pr): paginate review threads instead of reading only the first 100
• fix(pstack): respect fenced code blocks in plan checks #536 fix(pstack): respect fenced code blocks in plan checks
• fix(pstack): rename Comment Sicko agent id so Grok can spawn it #486 fix(pstack): rename Comment Sicko agent id so Grok can spawn it

#573 is included. #577 (fix(pstack): tell agents where bundled leaf skills live) is not included, because it is still open.

Verification

• git log 08bd59b..origin/main -- pstack/ lists the eight PRs above, and none of them changed the version.
• git grep 0.15.20 finds the two manifests only, and git grep 0.15.19 finds no pstack file.
• pstack/plugin.json passes the Agent Plugins v1.0.0 JSON schema. Its name, version, description, author, homepage, repository, license, and keywords match .cursor-plugin/plugin.json.
• node scripts/validate-plugins.mjs passes, as CI runs it.