fix: proper package entry point, missing pyproject modules, and updated install docs

[chenqiaoanying]

Summary

• Add proper package entry point — renamed internal async def main() to _main() and introduced a synchronous def main() as the [project.scripts] entry point so splunk-mcp-server works correctly when installed via pip or uv tool install
• Fix pyproject.toml modules — added guardrails and spl_risk_rules to py-modules so they are included in the installed package
• Fix FastMCP init param — changed description= to instructions= to match the current FastMCP API
• Expand installation docs — rewrote the installation section to cover three methods (uv local clone, uvx one-shot, pip from GitHub) and updated Claude Desktop and Claude Code configuration examples to use the installed splunk-mcp-server command or uvx instead of python server.py
• Add uv.lock — commit the lock file for reproducible installs

Test plan

• uv tool install --editable . then run splunk-mcp-server — server starts without error
• uvx --from "git+https://github.com/splunk/splunk-mcp-server2#subdirectory=python" splunk-mcp-server — runs directly without prior install
• pip install "git+https://github.com/splunk/splunk-mcp-server2#subdirectory=python" then splunk-mcp-server — server starts without error
• Verify guardrails and spl_risk_rules are importable after install
• Claude Desktop stdio config using splunk-mcp-server command connects successfully

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