docs: establish Go backend repository rules

AGENTS.md

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+# Backend Agent Operating Contract
+
+## Phase gate
+
+This repository is in specification mode. Do not create `go.mod`, application packages, migrations, containers, or executable code until the relevant capability spec is approved.
+
+For every Go-related task, always invoke and follow the `golang-how-to` skill, then the secondary Go skills it routes to. Before creating the Go project layout, ask the user to approve the architecture style and dependency-injection approach in that order.
+
+## Product invariants
+
+- The backend is the authority for authentication, authorization, tenant isolation, metric definitions, and data provenance.
+- Every merchant-scoped query derives merchant identity from the authenticated server-side session; never trust a client-supplied merchant identifier as authorization.
+- Cross-merchant results are minimum-cohort, anonymized aggregates only.
+- Use integer minor currency units or an explicitly documented fixed-precision representation; never floating point for money.
+- Preserve source timestamps and document their timezone assumptions. Normalize operational timestamps consistently.
+- Raw imports are immutable inputs; corrections are new import versions, not silent mutation.
+- `adjusted_fee` is obfuscated and may be used only for relative analysis.
+
+## Go baseline after approval
+
+- Keep `main` packages thin under `cmd/`; business code lives under `internal/`.
+- Use explicit constructors; avoid `init()` side effects and mutable globals.
+- Configuration comes from validated environment variables. Logs go to stdout as structured records without credentials, card keys, or sensitive payloads.
+- Propagate `context.Context`; bound external calls, queues, batches, and database pools.
+- Use parameterized queries and transactions with explicit boundaries. Stream/bulk-load the multi-million-row input instead of holding it in memory.
+- Passwords use an approved adaptive hash (Argon2id or bcrypt per the approved auth spec); sessions use secure, HttpOnly cookies and server-side revocation.
+- Required verification includes unit/integration tests, race detection where applicable, `go vet`, `golangci-lint`, `govulncheck`, and migration/import tests.
+
+## Git and documentation
+
+- Use atomic Conventional/Commitizen commits.
+- Track specs, ERD/schema definitions, migrations, importer code, validation rules, manifests/templates, and runbooks.
+- Ignore raw CSV archives, database volumes/dumps, credentials, and generated runtime data.
+- Changes to a metric, API, schema, authorization rule, or AI data boundary require the corresponding spec/ADR update.
+
+## AI boundary (post-MVP)
+
+AvalAI is accessed only from the backend using a server-held API key. Prefer the simplest bounded LLM call/workflow that satisfies the approved use case. Send minimized, sanitized aggregates with explicit budgets/timeouts; persist trace metadata and deterministic inputs, not unnecessary sensitive prompts. AI output is advisory and must cite underlying deterministic metrics.
+