docs: make AGENTS.md the home of design principles and documentation rules

[SaladDay]

Part of the documentation overhaul.

Changes

AGENTS.md becomes the single home of the project's design rules and documentation rules. CONTRIBUTING.md keeps how-to-work material. Nothing is stated in both files.

• Protocols at every boundary. Each component boundary has one protocol: one code file and one document. Protocols are deterministic: support is declared, never discovered through type assertions or name checks, and unsupported operations return typed errors. A table maps each boundary to its entry file and protocol document.
• Complexity stays in the adapter. A new Sandbox Provider, Harness, model provider or vendor feature changes only its adapter. It adds no vendor-specific Core path, store column, migration, deployment field, API field or Web UI. When the protocol cannot express a need, the protocol itself changes, in a change of its own.
• Public API. The pinned official protocol is the target. Native differences stay explicit in the coverage ledger. Applications reach Core only through the public contract.
• One home for each setting and datum. Each is written in one place and read from one place, with no fallback or alias. Configuration files are grouped by category. docs/configuration.md owns the layout.
• Pre-release: no compatibility layers.
• Known gaps. Short examples of current code that breaks these rules, so they are not copied.
• Documentation rules. One fact in one place; related material kept together; one audience and one job per document; plain, helpful tone; delete obsolete docs; no hard wraps; Web's page names; OPENAI_BASE_URL/OPENAI_API_KEY in examples.

In CONTRIBUTING.md, the decoupling, extension-contract, pre-release, documentation-rule and design/compatibility sections moved out. Evidence rules sit under Required checks. The ownership map points protocol boundaries and design rules to AGENTS.md.

docs/architecture.md keeps the overview and links to the AGENTS.md boundary table. docs/design-principles.md and contracts/agents-api/README.md link to AGENTS.md instead of restating its rules. The three files this PR owns are unwrapped: one line per paragraph, rendering identical.

Verification

• make check-names, make check-docs, make check-distribution
• Relative link and anchor check across the changed files: 0 broken
• Three rounds of independent blind review (Claude subagents). The third round's small findings were fixed and verified by the checks above.

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