chore(rfc): wrap markdown at 120 columns and add RFC style rules to the rfc skill

[johnthecat]

Tooling for writing and reviewing RFCs in this repo. Used to write the RFC stack #631 → #632 → #633 → #634, which ends in the Input Modality RFC.

Markdown formatting

.prettierrc wraps Markdown prose at 120 columns (proseWrap: always). Sentences in RFCs and design docs land on stable lines, so a review diff shows the sentence that changed rather than a rewrapped paragraph. Prettier is not part of CI; npm run format applies it and npm run format:check verifies it.

The rfc skill

The skill gains a Style section that turns the review rules for RFCs into something the skill applies while drafting:

• Cut: selling text, enumerations of examples, em-dash appositions, bold-lead mini-sections, self-references, transition narrative, restated sections, unvalidated defaults and their placeholder variables, metaphors with their own vocabulary.
• Shape: requirements as constraints on any acceptable design; one bullet is one rule; a design roadmap is a bullet list introduced by "The design has N parts:"; tables carry a one-line caption; Rust blocks are complete traits in the shape of rust/crates/truapi/src/api/*.rs; a shared concept is defined in one RFC and linked from the others; inline lists are examples unless declared exhaustive.
• Process: read the whole document first, trim in reading order, earlier text wins, fix invalidated terms in the same pass, verify no dangling references, and leave whole-section deletion to the author.

The mechanics section is reflowed to the new column width; its content is unchanged.

[TarikGul]

Good to have @filvecchiato review this.