[shinaoka]

Adds an explicit precedence rule, because agents follow whatever a document asserts.

What changed

• rules/index.md now opens with The Implementation Outranks The Documentation: the code, its tests and the CI configuration define current behavior, and a stale statement is a defect to fix rather than an instruction to follow.
• rules/common/docs-and-tests.md gains the operational procedure: verify against the code and CI, never edit code to satisfy stale prose, correct or file the document with a file:line reference, treat required reading, ownership pages and source of truth claims as high risk, and retire the describing documents in the same change as the implementation.

Why

Three concrete failures in this organisation came from prose that outlived its code:

• a design note still presented the removed CubeCL backend as the current GPU contract,
• an architecture page still assigned the primitive AD registry to tenferro-internal-ops, whose registry has no callers outside its own dead tree,
• a performance guide still described a thread pool API that the caller held engine had replaced.

Each one misdirected an agent that was following the rules correctly. Fixing the code was never the right response; correcting the documents was.

82 of the repositories under the organisation reference this index from their own AGENTS.md, so the rule reaches them without editing each repository.