docs: add .prettierrc and decide the README template shape for the manager

[ss-o]

Problem

Two Markdown hygiene gaps found during the zi repository-health evaluation of 2026-09-18:

1. The repository has no .prettierrc. runbooks/new-repository.md in z-shell/.github expects every repository to carry { "proseWrap": "never" } so Trunk's prettier keeps prose one paragraph per line (.github/instructions/documentation.instructions.md). The README is already written one paragraph per line, so this is a guard, not a reflow.
2. The root README.md follows a zi-specific shape (Quick start, Learn by task, Community and support, ...), not the organization README template's section map (templates/readme/zsh-plugin.md), and there is no docs/README.md although docs/ exists. zi is the plugin manager, not a plugin, so the template's Installation, Lifecycle, and Portable shell contract sections do not map one to one.

Decision needed

• Add .prettierrc (mechanical, its own pull request).
• Decide whether zi's README adopts the template structure with a docs/README.md (as annexes and tools did through README template guidance does not reach annex, tool, or module READMEs and conflicts with the .prettierrc runbook .github#624), or is recorded as the deliberate exception for the manager. Either way, document the outcome so the next README rewrite does not re-open the question.

[ss-o]

Project 28 triage pass (2026-10-02): stays in Triage.

• Premise check on next at 3fa8433 (premise confirmed): Item 1 is done: .prettierrc with { "proseWrap": "never" } is on next@3fa8433 and on main@4f2c0a7 (merged PR chore(docs): add .prettierrc with proseWrap never #541, promoted by chore: promote next to main #567). Item 2 is open: README.md keeps the zi-specific sections (Quick start, Learn by task, Community and support, ...), docs/ holds only CONTRIBUTING.md, images, man; no docs/README.md and no recorded decision on the template exception found in AGENTS.md, README.md or docs/.
• Proposed disposition: request more information.
• Unknowns and next investigation: Maintainer decision needed: adopt the organization README template with a docs/README.md, or record zi as the deliberate exception (where: AGENTS.md or docs/). Once decided the remaining work is a docs PR; the .prettierrc half can be noted as complete via PR chore(docs): add .prettierrc with proseWrap never #541.