docs: distribute package-specific GEMINI.md context to each package (#22734)

packages/core/GEMINI.md

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+# Gemini CLI Core (`@google/gemini-cli-core`)
+
+Backend logic for Gemini CLI: API orchestration, prompt construction, tool
+execution, and agent management.
+
+## Architecture
+
+- `src/agent/` & `src/agents/`: Agent lifecycle and sub-agent management.
+- `src/availability/`: Model availability checks.
+- `src/billing/`: Billing and usage tracking.
+- `src/code_assist/`: Code assistance features.
+- `src/commands/`: Built-in CLI command implementations.
+- `src/config/`: Configuration management.
+- `src/confirmation-bus/`: User confirmation flow for tool execution.
+- `src/core/`: Core types and shared logic.
+- `src/fallback/`: Fallback and retry strategies.
+- `src/hooks/`: Hook system for extensibility.
+- `src/ide/`: IDE integration interfaces.
+- `src/mcp/`: MCP (Model Context Protocol) client and server integration.
+- `src/output/`: Output formatting and rendering.
+- `src/policy/`: Policy enforcement (e.g., tool confirmation policies).
+- `src/prompts/`: System prompt construction and prompt snippets.
+- `src/resources/`: Resource management.
+- `src/routing/`: Model routing and selection logic.
+- `src/safety/`: Safety filtering and guardrails.
+- `src/scheduler/`: Task scheduling.
+- `src/services/`: Shared service layer.
+- `src/skills/`: Skill discovery and activation.
+- `src/telemetry/`: Usage telemetry and logging.
+- `src/tools/`: Built-in tool implementations (file system, shell, web, MCP).
+- `src/utils/`: Shared utility functions.
+- `src/voice/`: Voice input/output support.
+
+## Coding Conventions
+
+- **Legacy Snippets:** `src/prompts/snippets.legacy.ts` is a snapshot of an
+  older system prompt. Avoid changing the prompting verbiage to preserve its
+  historical behavior; however, structural changes to ensure compilation or
+  simplify the code are permitted.
+- **Style:** Follow existing backend logic patterns. This package has no UI
+  dependencies — keep it framework-agnostic.
+
+## Testing
+
+- Run tests: `npm test -w @google/gemini-cli-core`
+- Run a specific test:
+  `npm test -w @google/gemini-cli-core -- src/path/to/file.test.ts`