docs: add repository AGENTS.md guidance

AGENTS.md

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+# AGENTS.md
+
+## Project
+Household Telegram bot + mini app monorepo for shared rent/utilities/purchase accounting.
+
+## Core stack
+- Runtime/tooling: Bun
+- Language: TypeScript (strict)
+- Typecheck: tsgo (`@typescript/native-preview`)
+- Lint: Oxlint
+- Format: Oxfmt (no semicolons)
+- Bot: grammY
+- Mini app: SolidJS + Vite
+- Data platform: Supabase (planned)
+- Deploy: Cloud Run + Cloud Scheduler (planned)
+
+## Architecture
+Hexagonal architecture:
+- `packages/domain`: pure business rules/value objects only
+- `packages/application`: use-cases only
+- `packages/ports`: interfaces for external boundaries
+- `apps/*`: composition/wiring and delivery endpoints
+
+Boundary rules:
+- No framework/DB/HTTP imports in `packages/domain`
+- `packages/application` must not import adapter implementations
+- External SDK usage belongs outside domain/application core
+
+## Money and accounting rules
+- Never use floating-point money math
+- Store amounts in minor units
+- Deterministic split behavior only
+- Persist raw parsed purchase text + confidence + parser mode
+
+## Workflow
+- Work from Linear tickets and linked specs in `docs/specs/`
+- One ticket at a time, small commits
+- Before implementation: re-check ticket/spec and assumptions
+- Do not commit without explicit user approval
+- Run CodeRabbit review before merge (`bun run review:coderabbit`)
+
+## Quality gates
+Required before PR/merge:
+- `bun run format:check`
+- `bun run lint`
+- `bun run typecheck`
+- `bun run test`
+- `bun run build`
+
+## CI/CD
+- CI workflow runs parallel quality jobs on push/PR to `main`
+- CD workflow deploys on successful `main` CI or manual trigger
+- Required CD secrets:
+  - `GCP_PROJECT_ID`
+  - `GCP_WORKLOAD_IDENTITY_PROVIDER`
+  - `GCP_SERVICE_ACCOUNT`
+
+## Docs as source of truth
+- Roadmap: `docs/roadmap.md`
+- Specs template: `docs/specs/README.md`
+- ADRs: `docs/decisions/*`
+- Runbooks: `docs/runbooks/*`