chore: remove superpowers analysis spec

[parlakisik]

ctx must be self-contained — no external plugin libraries.

Deleted specs/superpowers-analysis.md.

All equivalent capabilities already exist natively in ctx.

[josealekhine]

The PR's assessment is partially right but the approach is wrong.

1. Correct: The superpowers plugin is external, and ctx already has native equivalents for every capability it provides. The spec itself acknowledges this —
   it's a comparison document, not a dependency.

2. Incorrect framing: The spec isn't a dependency on superpowers — it was deliberately created in e6d8088 as an analysis document when the docs/superpowers/
   directory (which contained actual external skill files) was deleted. The spec captures what patterns were worth noting from the external project. It's areference, not a liability.

3. The PR body says "All equivalent capabilities already exist natively in ctx" —
   which is true, but the spec already says exactly that. Deleting the analysis doesn't make ctx more self-contained; the external code was already removed.

I'm merging this nonetheless.

Will create a more refined spec later.

Thanks.