feat: add gemini install target

.gemini/GEMINI.md

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+# ECC for Gemini CLI
+
+This file provides Gemini CLI with the baseline ECC workflow, review standards, and security checks for repositories that install the Gemini target.
+
+## Overview
+
+Everything Claude Code (ECC) is a cross-harness coding system with 36 specialized agents, 142 skills, and 68 commands.
+
+Gemini support is currently focused on a strong project-local instruction layer via `.gemini/GEMINI.md`, plus the shared MCP catalog and package-manager setup assets shipped by the installer.
+
+## Core Workflow
+
+1. Plan before editing large features.
+2. Prefer test-first changes for bug fixes and new functionality.
+3. Review for security before shipping.
+4. Keep changes self-contained, readable, and easy to revert.
+
+## Coding Standards
+
+- Prefer immutable updates over in-place mutation.
+- Keep functions small and files focused.
+- Validate user input at boundaries.
+- Never hardcode secrets.
+- Fail loudly with clear error messages instead of silently swallowing problems.
+
+## Security Checklist
+
+Before any commit:
+
+- No hardcoded API keys, passwords, or tokens
+- All external input validated
+- Parameterized queries for database writes
+- Sanitized HTML output where applicable
+- Authz/authn checked for sensitive paths
+- Error messages scrubbed of sensitive internals
+
+## Delivery Standards
+
+- Use conventional commits: `feat`, `fix`, `refactor`, `docs`, `test`, `chore`, `perf`, `ci`
+- Run targeted verification for touched areas before shipping
+- Prefer contained local implementations over adding new third-party runtime dependencies
+
+## ECC Areas To Reuse
+
+- `AGENTS.md` for repo-wide operating rules
+- `skills/` for deep workflow guidance
+- `commands/` for slash-command patterns worth adapting into prompts/macros
+- `mcp-configs/` for shared connector baselines