chore(skills): cut the rfc skill to a 1-2 pager

[filvecchiato]

The rfc skill targets a 1-2 page RFC that settles the approach and leaves the
details to the implementer.

196 → 66 lines. Gone: the mandatory rounds of clarifying questions, the ten-area
clarity checklist, the hand-it-back-for-review phase, and the Polkadot Fellows
template's Stakeholders, Testing/Security/Privacy,
Performance/Ergonomics/Compatibility, Prior Art and Future Directions sections.
Drawbacks and Alternatives merge into one Trade-offs section.

Template now matches number-rfc.yml

The old template did not, in three ways worth knowing about independently of the
brevity change:

• The H1 is # RFC — Title with an em dash, which is the exact form the
  numbering step rewrites (s/^# RFC — /# RFC NNNN — /). # RFC: Feature Name Here matched neither pattern, so the assigned number was never injected into
  the title.
• Frontmatter carries title / owner / status where extract_meta reads
  them, and status: draft is set explicitly: a missing status is defaulted to
  accepted, so a draft would have been indexed as accepted.
• The four-row metadata table is dropped, since extract_meta prefers
  frontmatter. That also removes a hardcoded author name that would have
  mis-attributed every RFC generated from the template.

The skill now says not to number the file and not to touch _index.md — CI does
both on merge to main.

Not addressed here

check-rfc.yml fails any PR touching docs/rfcs/** that does not also change
rust/crates/truapi/. A host-side proposal cannot satisfy that, since the crate
invariants keep host-side runtime types in truapi-platform/truapi-server, so
such a proposal has to be filed under docs/features/ instead. The skill
documents the constraint; removing it is a separate call about the process rather
than the skill.