[SaladDay]

Part 1 of 3 of the documentation cleanup.

New-user path

• README (en/zh): what the project is, a four-step install, the two APIs, and a "what do you want to do" documentation table.
• Installation: the default path comes first: prerequisites → install → make Core reachable → sign in → next steps. Backends, modes, split deployment and the directory layout move under "Advanced installation". Flags stay owned by install-options.md.
• Quickstart: three steps from a Project API key to a finished Session. The model provider is optional when the installation has a default model.
• User guide: task-oriented. It is now the single owner of Environment kinds, harness and model choice, and model provider precedence, including the unified model configuration (harness_config, and the model can be omitted for inline hosted Sessions).

One owner per topic

Each topic below now has one owner document; other places link to it instead of restating it.

Topic	Owner
Namespaces and credentials	docs/api/README.md
Projects and keys, administrator authority, isolation	docs/design-principles.md
Key-rotation steps	operations.md
Backend and size changes	configuration.md
Executor credential rotation	self-hosted.md
Model precedence	user-guide.md

Validation

Run on the final stacked head (#226):

• make check-docs passes.
• check-names passes.
• A link and anchor check over all tracked Markdown passes.

#226 (API guide and examples) adds anchors this PR links to. Merge the two back to back.

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