Document homepage architecture and CSS guardrails for OASIS site edits

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Motivation

• Capture the homepage/hero/sidebar/header layout decisions and the “rules of the road” discovered during recent iterations so future edits do not reintroduce regressions (white gap, hero/sidebar overlap, brittle selectors).
• Provide a single canonical, machine- and human-readable source for safe homepage edits and Codex prompts so automated agents and contributors follow the same constraints.

Description

• Add a canonical developer guide at docs/dev/oasis-site-dev-guide.md that documents the two-column architecture (.oasis-layout → .oasis-sidebar + .oasis-main), the hero placement contract, sidebar sizing via --oasis-sidebar-w, the white-gap root cause and container resets, selector guidance (no :has()), file ownership rules (docs/styles/extra.css = brand skin, docs/assets/css/style.css = structural glue), a debug playbook, and 3–5 Codex-ready prompts.
• Link the new guide from README.md and surface it in CONTRIBUTING.md so contributors are pointed to the single source of truth before editing homepage structure or CSS.
• Add inline guard comments and ownership reminders: a homepage structure guard in docs/index.md, a visual-authority header in docs/styles/extra.css, and a structural-glue header in docs/assets/css/style.css to reduce ambiguity about where to place aesthetic vs structural rules.
• Make only documentation and comment-level edits; no runtime/layout code changes were introduced beyond comments and the new guide (the guide references existing CSS rules such as .oasis-layout .oasis-main .oasis-hero and the .oasis-layout .md-content__inner reset).

Testing

• Built the site with python -m mkdocs build and the build completed successfully.
• The build emitted unrelated repository warnings about nav/link hygiene and new-file git timestamps, but no errors related to the doc or CSS changes were observed.

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Codex Task

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