pstack: bump to 0.9.1 and list the hillclimb playbook in the README

[poteto]

Summary

Housekeeping follow-up to #132, which added the Hillclimb playbook but did not touch the version or README.

• Bump the plugin patch version 0.9.0 -> 0.9.1.
• README: count goes from fifteen to sixteen playbooks, and a hillclimb row is added next to perf.

Test plan

• README playbook table now lists hillclimb; count matches.

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Note

Low Risk
Documentation and version metadata only; no runtime or skill behavior changes in this diff.

Overview
Release housekeeping after the Hillclimb playbook landed elsewhere: bumps the pstack plugin version from 0.9.0 to 0.9.1 in plugin.json.

The README now matches shipped playbooks: the /poteto-mode count goes from fifteen to sixteen, and a new hillclimb row is added (after perf) describing sustained, measured improvement of one metric with one commit per accepted win.

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