[Feature]: Non-interactive one-shot mode — gcode -p "question"

[shauryagangrade]

Problem

GCode is interactive-only. Scripts, CI, and editors can't ask it a single
question and get an answer without driving a pty.

Proposed approach

• Add gcode --prompt "..." (short -p) that runs one turn in the current
  session, prints the reply (optionally --json), and exits.
• No banner, no slash menu, no interactive prompt; still load .gcoderc,
  env, and tools. Non-tty safe: auto-reject bash unless --yes.
• Keep --session semantics (reads/updates the session history).

Where to look

• gcode/cli.py:239-... (main) — argument parsing and startup flow.
• gcode/agent.py:101-137 (run_turn) for the single-turn loop.

Acceptance criteria

• gcode -p "summarize this repo" prints a reply and exits 0.
• Works non-interactively (CI), and without an API key for ollama models.

Difficulty

Medium — good stepping-stone issue.