docs: add repository agent guidance

[frahlg]

What changed

Adds the repository-level AGENTS.md with Miranda's naming, layout, and non-negotiable security/interop invariants.

Why

Future automated changes should preserve the blind-relay boundary, Go/JS byte compatibility, passkey-derived owner identity, and the existing Go module path.

Impact

Documentation and contributor guidance only; no runtime or release artifacts change.

Validation

• staged diff contains only AGENTS.md
• git diff --cached --check passed
• no runtime tests required for a documentation-only change

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Broaden vector regeneration guidance

For changes to pairing or owner-identity derivation, this command only regenerates the Noise KK and legacy owner vectors from go/internal/noise/interop_test.go; other byte-compatibility fixtures are generated by separate tests such as go/internal/pairing/interop_test.go::TestPairInteropVectorsStable and go/internal/identity/signer_test.go::TestIdentityDerivationVector. If an agent follows this AGENTS.md instruction after a legitimate change in those areas, it will leave stale testdata and break the Go/JS parity tests, so this should either list the relevant vector update commands or describe running the package-specific generator for the code being changed.

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