Fix cobra command and rename more things from cagent to docker agent

[dgageot]

• Fix completion
• Remove special case for cagent and cagent.exe
• Fix error message when a gateway is set but Docker Desktop is not started
• Fix the categories of the sub commands in the usage message
• Fix the cobra Examples
• Simplify the code especially for the handling of the default "run" command

[docker-agent]

Review Summary

Assessment: 🟢 APPROVE

This PR successfully renames 'cagent' to 'docker-agent' throughout the codebase and fixes Cobra command configuration. The changes are primarily cosmetic (renaming) with one notable behavior change: the root command now defaults to running the 'run' subcommand when invoked without arguments, instead of showing help.

Key Changes:

• Removed deprecated binary name (cagent) from build configuration
• Updated all CLI examples and documentation strings
• Consolidated telemetry initialization into PersistentPreRunE
• Changed root command default behavior to execute run subcommand
• Simplified Cobra command structure by removing runtime binary name detection

Verification:
All changes reviewed. No bugs detected in the modified code. The code handles edge cases appropriately (e.g., the 'run' command is guaranteed to exist when Find() is called).

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No issues found in the changed code.