Expose OBOT_BASE_URL as global flag in CLI help

[Copilot]

The OBOT_BASE_URL environment variable was not discoverable in CLI help output, making it difficult for non-interactive workflows to determine how to configure the base URL.

Changes

• Added BaseURL field to Obot struct with usage tag and default value
• Updated PersistentPre to apply the BaseURL value to the API client
• Initialized BaseURL from OBOT_BASE_URL environment variable in New()

The flag now appears in all command help outputs:

Global Flags:
      --base-url string   Base URL for the OBOT API ($OBOT_BASE_URL) (default "http://localhost:8080/api")
      --debug             Enable debug logging ($OBOT_DEBUG)

The environment variable continues to work as before, with the flag providing an additional explicit configuration method.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Expose OBOT_BASE_URL in cli help</issue_title>
<issue_description>We've have some light interest in hitting obot from non-human workflows. Generally people need a token for this and obot token should be sufficient, but the OBOT_BASE_URL env var is not discoveable in the cli help:

obot token --help
Usage:
  obot token [flags]

Flags:
      --force-refresh   Force refresh the token ($OBOT_TOKEN_FORCE_REFRESH)
  -h, --help            help for token
      --no-expiration   Set the token to never expire ($OBOT_TOKEN_NO_EXPIRATION)

Global Flags:
      --debug   Enable debug logging ($OBOT_DEBUG)

id for it to be a global flag that gets exposed in the help </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Expose OBOT_BASE_URL in cli help #5263

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[cjellick]

Why did you have to move BaseURL onto this struct rather than just leveraging the one thats already on Client?