feat(gemini): add Gemini CLI as sixth target provider

Add `--to gemini` support for both `convert` and `install` commands,
converting Claude Code plugins into Gemini CLI-compatible format.

- Agents convert to `.gemini/skills/*/SKILL.md` with description frontmatter
- Commands convert to `.gemini/commands/*.toml` with TOML prompt format
- Namespaced commands create directory structure (workflows:plan -> workflows/plan.toml)
- Skills pass through unchanged (identical SKILL.md standard)
- MCP servers written to `.gemini/settings.json` with merge support
- Content transforms: .claude/ paths, Task calls, @agent references
- Hooks emit warning (different format in Gemini)

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

src/types/gemini.ts

@@ -0,0 +1,27 @@
+export type GeminiSkill = {
+  name: string
+  content: string // Full SKILL.md with YAML frontmatter
+}
+
+export type GeminiSkillDir = {
+  name: string
+  sourceDir: string
+}
+
+export type GeminiCommand = {
+  name: string // e.g. "plan" or "workflows/plan"
+  content: string // Full TOML content
+}
+
+export type GeminiBundle = {
+  generatedSkills: GeminiSkill[] // From agents
+  skillDirs: GeminiSkillDir[] // From skills (pass-through)
+  commands: GeminiCommand[]
+  mcpServers?: Record<string, {
+    command?: string
+    args?: string[]
+    env?: Record<string, string>
+    url?: string
+    headers?: Record<string, string>
+  }>
+}