pstack: bump version to 0.9.0

[poteto]

Summary

• Bumps the plugin from 0.8.0 to 0.9.0 for the recent poteto-mode work now on main: the sequence-verifiable-units principle (pstack: add 'sequence work into verifiable units' principle #123), the self-unblock empirical-fork trigger (pstack: self-unblock empirical forks with a sketch instead of asking #122), and the prototype and refactoring playbook fixes (pstack: shape refactoring commits before opening the PR #124).

Delivery

One commit, the version field only.

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Note

Low Risk
Metadata-only version field change with no runtime or behavioral code modifications.

Overview
Bumps the pstack Cursor plugin release from 0.8.0 to 0.9.0 in plugin.json only—no skill, agent, or workflow file changes in this PR.

The version aligns the published plugin with recent poteto-mode updates already on main (sequence-verifiable units, self-unblock empirical-fork trigger, prototype/refactoring playbook fixes).

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