Summary

Align the gro codebase with engineering standards for consistency, mechanical enforcement, and maintainability. This is a multi-commit refactoring effort covering architecture enforcement, test infrastructure, documentation, and bug fixes.

Changes

Architecture & Enforcement

• Add structural tests (internal/architecture/architecture_test.go) that mechanically enforce codebase patterns at CI time: client interfaces, ClientFactory DI, NewCommand() factories, --json on leaf commands, read-only scopes, dependency direction
• Add coverage gate to CI (60% threshold)

Code Standards Alignment

• Propagate context.Context through all API client methods and command handlers
• Relocate interfaces from implementation packages to consumer packages (interface-at-consumer pattern)
• Remove deprecated gmail wrapper functions
• Use present participle in error messages per Go conventions
• Add signal handling with context propagation

Test Infrastructure

• Remove testify dependency, replace with stdlib-based assertion helpers (internal/testutil/assert.go)
• Centralize CaptureStdout and generic WithFactory[T] helpers (internal/testutil/helpers.go)
• Add t.Parallel() to safe test files
• Add function-field mock structs with compile-time interface checks in each domain

Documentation

• Restructure CLAUDE.md as a concise table of contents (~90 lines, down from ~350)
• Create docs/architecture.md, docs/golden-principles.md, docs/adding-a-domain.md
• Add package doc comments to all packages

Bug Fixes

• Fix --json flag ignored on empty results across all list/search commands (output [] instead of plaintext)

Build & Lint

• Align Makefile targets and CI workflow
• Clean up stale lint suppressions in .golangci.yml

Verification

• make check passes (tidy, lint, test, build)
• make test-cover-check passes (63.6% coverage, 60% threshold)
• All 12 structural tests pass
• Integration tests pass against live Google APIs