docs: update install and test instructions in README

[Coding-Dev-Tools]

Replaces template boilerplate npm install/test sections with working pip git+ install and pytest instructions.

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[github-actions]

🤖 Automated Code Review

✅ Ruff Lint — No issues

⚠️ Ruff Format — Formatting needed

unformatted: File would be reformatted
   --> src/api_contract_guardian/cli.py:21:19
    |
20  |
    -     warnings.warn(
    -         "revenueholdings-license not installed; license checks skipped", stacklevel=2
    -     )
21  +     warnings.warn("revenueholdings-license not installed; license checks skipped", stacklevel=2)
22  |     _has_rh = False
--------------------------------------------------------------------------------
53  |
    - def _validate_output_format(
    -     format_name: str, allowed: tuple[str, ...], command: str
    - ) -> str:
54  + def _validate_output_format(format_name: str, allowed: tuple[str, ...], command: str) -> str:
55  |     """Reject unsupported output formats before the command runs."""
56  |     if format_name not in allowed:
57  |         allowed_list = ", ".join(allowed)
    -         raise typer.BadParameter(
    -             f"Unsupported {command} format '{format_name}'. Choose from: {allowed_list}"
    -         )
58  +         raise typer.BadParameter(f"Unsupported {command} format '{format_name}'. Choose from: {allowed_list}")
59  |     return format_name
--------------------------------------------------------------------------------
93  |     global _require_license_strict
    -     _require_license_strict = require_license_flag or bool(
    -         os.environ.get("REVENUEHOLDINGS_REQUIRE_LICENSE")
    -     )
94  +     _require_license_strict = require_license_flag or bool(os.environ.get("REVENUEHOLDINGS_REQUIRE_LICENSE"))
95  |

✅ Secret Detection — Clean

✅ Large Files — Within limits

📊 Diff Stats — 1 file(s) changed

 README.md | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

---

Verdict: ⚠️ Warnings — Lint/format issues found. Recommend fixing before merge.

Automated by Coding-Dev-Tools/.github reusable workflow.