chore(pstack): bump to 0.15.21

[poteto]

Why

This is a patch release of pstack. It ships the one pstack change that merged after the last version bump (#581, 0.15.19 → 0.15.20). This PR changes only the version. It does not change behavior.

Scope

• pstack/.cursor-plugin/plugin.json. Version 0.15.20 → 0.15.21.
• pstack/plugin.json (the Agent Plugins v1 root manifest). Version 0.15.20 → 0.15.21. 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.

Included PRs

• fix(pstack): tell agents where bundled leaf skills live #577 fix(pstack): tell agents where bundled leaf skills live

#580 also merged after #581, but it does not touch pstack.

Verification

• git log 1b6d44f..origin/main -- pstack/ lists only fix(pstack): tell agents where bundled leaf skills live #577, and fix(pstack): tell agents where bundled leaf skills live #577 does not change the version.
• git grep 0.15.21 finds the two manifests only, and no pstack file still has 0.15.20.
• 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.