feat: add Gemini CLI backend support

[CCALITA]

Summary

Add Gemini CLI (gemini) as a new backend for inline-cli, enabling users to route queries through the Google Gemini CLI.

• New internal/backend/gemini.go — implements the Backend interface by invoking gemini -p <prompt> -o text and streaming plain text stdout
• Lazy binary resolution (resolves PATH on each query, not at construction)
• Does not pass --model — lets gemini use its own configured default
• Captures stderr on failure to surface meaningful error messages instead of generic "exit status 1"
• Conversation history is prepended to the prompt for multi-turn context
• Config: gemini_path in TOML / INLINE_CLI_GEMINI_PATH env var for custom binary path
• Wired "gemini" case into createBackend() in internal/daemon/server.go

Files changed

File	Change
internal/backend/gemini.go	New backend implementation with stderr capture
internal/backend/gemini_test.go	9 tests: constructor, PATH resolution, query, history, error handling with/without stderr
internal/config/config.go	Added GeminiPath field and env var override
internal/daemon/server.go	Added "gemini" case to createBackend()

Usage

# ~/.inline-cli/config.toml
backend = "gemini"
# gemini_path = "/custom/path/to/gemini"  # optional

Or via environment variable:

export INLINE_CLI_BACKEND=gemini

Test plan

• go build ./... passes
• go test ./... -race passes (9 backend tests)
• Verified correct CLI flags: gemini -p <prompt> -o text
• Does not pass invalid model string to gemini
• Stderr captured and surfaced in error messages