feat: use detached spawn for agent

[sgammon]

Summary

Adds functionality to detect and use an existing agent, if available. Adds several unit tests, and shifts to spawning the agent via a detached child process rather than calling buildless agent start.

Changelog

• feat: detect and use existing agent ("unmanaged" mode)
• fix: prepare temp paths before installation
• test: significant improvement in test coverage

[codecov]

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[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
26 Code Smells

32.9% Coverage
0.0% Duplication

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