Revise README for improved clarity on Pstack features

[arjitj2]

Updated the README to clarify the capabilities of Pstack and its coordination process.

Closes #

What changed

Verification

• Bun tests, strict typecheck, static invariants, and plugin validation pass.
• For plugin changes, the exact candidate is installed in every affected harness.
• The changed behavior passes from each affected user surface.
• Evidence below names the version or commit, action, and observed result.
• Maintenance-only changes prove plugin-tree equality and exercise the changed CLI or workflow.

Live evidence:

Documentation impact

Owner doc updated at , or none, <reason> when there is no user-facing change. See the documentation ownership map.

Use installed-host evidence for plugin changes and actual CLI or workflow evidence for maintenance-only changes. A pull request without applicable live evidence remains a draft. Do not merge, tag, release, or roll it out.