IMP-063: Explain public docs through requirements and composition

[irl-dan]

The docs currently define OpenProse primarily as standing outcomes and tell agent readers to use retired Services / Ensures syntax. This change explains contract authoring as expressing intent by composing requirements, including required results, steps, limits, discretion, and evidence.

• Update all seven MDX pages, metadata, navigation copy, README, and the public getting-started skill. Keep the /declare-outcomes URL and aliases for changed heading anchors.
• Identify the documented public skill as 0.18.0 (runtime_contract: 2) at openprose/prose@770cebc9e03a7150a73bd454b7a6da307595ff9d. Retain its five kinds, Forme/VM/ProseScript roles, and syntax. Fenced MDX examples and vendored source bytes are unchanged; older snapshot headings are explicitly identified with a link to current sources.
• Distinguish requirements from host capabilities, a receipt from verified satisfaction, and the language specification from the reference harness 0.3.3 gaps recorded there. Correct the optional source identity explanation from the public format reference.
• Update the public skill manifest's description, canonical docs.prose.md URL, and exact content digest, together with its generation source and existing URL check. README now identifies the retained legacy .prose maintenance programs and the model-based prebuild hook accurately.

This is an editorial and onboarding change. The agent-readable getting-started guide now teaches existing 0.18 syntax, which affects guidance consumed by readers, but adds no runtime implementation, grammar, roles, or compatibility promise. No release, deployment, consumer upgrade, or model execution was performed.

Validation at 55772b50c722233fef40b538be7f70548b7aec3c, based on main@6b0f7dc13b2613f6548a45c96b35889802b6a2cb:

• Repository-pinned pnpm 10.34.5 frozen install; typecheck, lint, spell, punctuation, and internal link checks passed (28 links inspected).
• All 93 existing tests passed in 11 files.
• CI/Docker build path pnpm exec next build passed in public and preview modes; standalone HTTP smoke checks passed 44/44 public and 42/42 preview.
• Manifest SHA-256 matches the updated skill bytes. All fenced MDX examples and the vendored subtree match base.
• Rendered the changed public introductions and references in Chrome; independent review checked the authoring page, preserved anchors, and public source references.

Builds retain the existing Turbopack whole-project tracing warning. The initial host-default pnpm 11 install was incompatible with the repository's script-approval setup; its incidental config edit was reverted and checks used pnpm 10.34.5. The model-based pnpm build prebuild hook was not run: it invokes retained legacy maintenance programs requiring a separate compatibility update. Static/site checks do not establish OpenProse execution equivalence or close the documented harness gaps.

Task: IMP-063. Companions: public Site copy, company Site, core authoring reference. The public skill companion is openprose/prose#186. This docs change is based directly on current main and does not require those PRs for a successful build.

Final Verify and CodeQL checks passed. Independent review at 55772b50c722233fef40b538be7f70548b7aec3c found no unresolved findings, including focused test, digest, preserved-example, link and rendering checks. Retained review record (workspace access required).