[Feature]: --cwd DIR flag to start a session in a given directory

[shauryagangrade]

Problem

GCode always operates in the directory it was launched from. There's no way
to point it at another project without cd (and switching back), which
complicates editor/alias integration.

Proposed approach

• Add gcode --cwd DIR (like -C in git) that os.chdirs before loading
  config/.gcoderc and tools, so the banner and all tools reflect DIR.
• Resolve .gcoderc lookup against the chosen directory.

Where to look

• gcode/cli.py:239-... (main) — startup order (env → config → model → tools).
• gcode/config.py load_config(project_root=...) (in feat: add optional .gcoderc configuration file support #48).

Acceptance criteria

• gcode --cwd /some/project shows /some/project in the banner and tool calls operate there.
• .gcoderc in that directory is honored.

Difficulty

Low/Medium — good first issue.