Document Linux agent installer pool behavior

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Summary

• Added setup guidance for the automated Linux agent installer, including M2M token usage, pool name/ID values, quoting pool names with spaces, client-only installs, and passing service flags.
• Added troubleshooting guidance for cases where the Linux installer does not create or start the agent service.

Source Verification

• Verified against the accessible Juice-Labs/juice-oss installer source and recent merged installer change for quoted pool names.
• The private Juice-Labs/juice repository was not resolvable from this agent, so source verification used juice-oss codepaths that publish the installer behavior.

Codepaths Covered

• installer/install-juice.sh: INSTALL_JUICE_TOKEN, INSTALL_JUICE_POOL, client-only install behavior, service creation, and service argument forwarding.

Knowledge Gaps Addressed

• Clarifies that pool values can be names or IDs.
• Calls out quoting requirements for pool names containing spaces.
• Explains why omitting INSTALL_JUICE_POOL skips agent service creation/startup.