feat: add optional .gcoderc configuration file support

[aryansk]

Problem

Closes #15. GCode settings (default model, auto-approve, tool behavior) were
only configurable via command-line flags or environment variables — there was
no persistent file a user could edit across sessions.

Changes

• New gcode/config.py: dependency-free loader for a .gcoderc file in the
  project root (or ~/.gcode/.gcoderc for user-wide defaults). Simple
  key = value format with # comments; bool/int/string values are parsed.
• Supported keys: model, auto_approve, bash_timeout, system_prompt.
• Precedence is documented and enforced: CLI flag / env var > config file >
  built-in default (--model/GCODE_MODEL beat model, --yes beats
  auto_approve).
• gcode/tools.py: set_bash_timeout() lets the config override the bash
  tool's 300s default.
• gcode/cli.py: startup loads the config, resolves the model, auto-approve,
  timeout, and system prompt (used for new and cleared sessions).
• README: new "Configuration file" section with a table of supported keys.

Validation

• pytest: 53 passed (7 new tests covering parsing, precedence, and
  fallbacks)
• ruff check / ruff format --check / mypy gcode / compileall: all clean
• Live: a sample .gcoderc is parsed and its model resolved correctly

[shauryagangrade]

Hi — reviewing this. Two things before it can merge:

1. Stale branch + merge conflict. The branch is based on e4e062e, before fix: make common errors actionable with next-step hints #47 landed. Please rebase (or merge main) in. The only conflict is a trivial import-block collision in gcode/cli.py — both #47s merge and this branch add imports at the top of the same block. Fix: keep both import blocks (the gcode.config one from this branch and the gcode.errors one from fix: make common errors actionable with next-step hints #47). Everything else merges cleanly.

2. Docstring nit (not blocking): gcode/config.py module docstring says it reads ~/.gcode/config, but the code and PR body use ~/.gcode/.gcoderc (USER_CONFIG_PATH). Minor inconsistency — worth aligning.

Nice, focused feature otherwise — clean precedence rules, good test coverage. cc @aryansk

[aryansk]

Both points are addressed in the merged head 54faa77: the branch was rebased onto current main (keeping both import blocks — gcode.config from this change and gcode.errors from #47), and the module docstring now reads ~/.gcode/.gcoderc to match USER_CONFIG_PATH. Thanks for the review and for merging!