docs: refresh CONTRIBUTING with the real dev workflow

[aryansk]

Problem

CONTRIBUTING.md describes pip install -e . only and says nothing about the
actual stack: uv sync, ruff, mypy, bandit, the CI matrix, or how to add a
new tool/slash command. New contributors can't validate their work locally
without reverse-engineering CI. Closes #73.

Change

Rewrites CONTRIBUTING.md to match the real dev workflow:

• uv sync --extra dev setup with a pointer to the Windows guide.
• The exact commands CI runs (ruff check, ruff format --check,
  mypy gcode, bandit, compileall, pytest) and the Python
  3.10–3.13 matrix note.
• An end-to-end checklist for adding an agent tool (gcode/tools.py,
  ALL_TOOLS, TOOL_MAP, tests) and one for adding a slash command
  (gcode/cli.py handler, _SLASH_COMMANDS in gcode/ui.py,
  _print_help, README Commands list, tests).
• Correct issue-template links (bug_report.yml, feature_request.yml).

Why this approach

Docs-only, no behavior change. The checklists were verified against the
current source (tools.py, ui.py, cli.py, ci.yml) so a newcomer can go from
clone to green local checks using the doc alone.

Testing

• git diff --check clean.
• Every internal link/anchor verified against the repo tree
  (README #setup/#use, .github/ISSUE_TEMPLATE/*.yml, docs/windows.md).
• Commands mirror .github/workflows/ci.yml exactly.

Documentation and release impact

• User-facing documentation updated
• Changelog/release note needed
• Migration or compatibility note needed
• No documentation impact

Review notes

• Known limitations: none.
• Merge note: draft PR chore: add pre-commit config mirroring the CI lint job #85 (pre-commit config) also edits CONTRIBUTING.md
  (adds a Pre-commit Hooks section); this PR fully rewrites the file. If both
  merge, a small rebase on CONTRIBUTING.md is needed — happy to do it.
• Follow-up issue: none.