docs: add Gemini CLI spec and update README with gemini target

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/specs/gemini.md

@@ -0,0 +1,122 @@
+# Gemini CLI Spec (GEMINI.md, Commands, Skills, MCP, Settings)
+
+Last verified: 2026-02-14
+
+## Primary sources
+
+```
+https://github.com/google-gemini/gemini-cli
+https://geminicli.com/docs/get-started/configuration/
+https://geminicli.com/docs/cli/custom-commands/
+https://geminicli.com/docs/cli/skills/
+https://geminicli.com/docs/cli/creating-skills/
+https://geminicli.com/docs/extensions/writing-extensions/
+https://google-gemini.github.io/gemini-cli/docs/tools/mcp-server.html
+```
+
+## Config locations
+
+- User-level config: `~/.gemini/settings.json`
+- Project-level config: `.gemini/settings.json`
+- Project-level takes precedence over user-level for most settings.
+- GEMINI.md context file lives at project root (similar to CLAUDE.md).
+
+## GEMINI.md context file
+
+- A markdown file at project root loaded into every session's context.
+- Used for project-wide instructions, coding standards, and conventions.
+- Equivalent to Claude Code's CLAUDE.md.
+
+## Custom commands (TOML format)
+
+- Custom commands are TOML files stored in `.gemini/commands/`.
+- Command name is derived from the file path: `.gemini/commands/git/commit.toml` becomes `/git:commit`.
+- Directory-based namespacing: subdirectories create namespaced commands.
+- Each command file has two fields:
+  - `description` (string): One-line description shown in `/help`
+  - `prompt` (string): The prompt sent to the model
+- Supports placeholders:
+  - `{{args}}` — user-provided arguments
+  - `!{shell}` — output of a shell command
+  - `@{file}` — contents of a file
+- Example:
+
+```toml
+description = "Create a git commit with a good message"
+prompt = """
+Look at the current git diff and create a commit with a descriptive message.
+
+User request: {{args}}
+"""
+```
+
+## Skills (SKILL.md standard)
+
+- A skill is a folder containing `SKILL.md` plus optional supporting files.
+- Skills live in `.gemini/skills/`.
+- `SKILL.md` uses YAML frontmatter with `name` and `description` fields.
+- Gemini activates skills on demand via `activate_skill` tool based on description matching.
+- The `description` field is critical — Gemini uses it to decide when to activate the skill.
+- Format is identical to Claude Code's SKILL.md standard.
+- Example:
+
+```yaml
+---
+name: security-reviewer
+description: Review code for security vulnerabilities and OWASP compliance
+---
+
+# Security Reviewer
+
+Detailed instructions for security review...
+```
+
+## MCP server configuration
+
+- MCP servers are configured in `settings.json` under the `mcpServers` key.
+- Same MCP protocol as Claude Code; different config location.
+- Supports `command`, `args`, `env` for stdio transport.
+- Supports `url`, `headers` for HTTP/SSE transport.
+- Additional Gemini-specific fields: `cwd`, `timeout`, `trust`, `includeTools`, `excludeTools`.
+- Example:
+
+```json
+{
+  "mcpServers": {
+    "context7": {
+      "url": "https://mcp.context7.com/mcp"
+    },
+    "playwright": {
+      "command": "npx",
+      "args": ["-y", "@anthropic/mcp-playwright"]
+    }
+  }
+}
+```
+
+## Hooks
+
+- Gemini supports hooks: `BeforeTool`, `AfterTool`, `SessionStart`, etc.
+- Hooks use a different format from Claude Code hooks (matchers-based).
+- Not converted by the plugin converter — a warning is emitted.
+
+## Extensions
+
+- Extensions are distributable packages for Gemini CLI.
+- They extend functionality with custom tools, hooks, and commands.
+- Not used for plugin conversion (different purpose from Claude Code plugins).
+
+## Settings.json structure
+
+```json
+{
+  "model": "gemini-2.5-pro",
+  "mcpServers": { ... },
+  "tools": {
+    "sandbox": true
+  }
+}
+```
+
+- Only the `mcpServers` key is written during plugin conversion.
+- Other settings (model, tools, sandbox) are user-specific and out of scope.