docs: add Premium license warning for external_agent resource (#434)

* docs: add Premium license warning for external_agent resource

Co-authored-by: M Atif Ali <[email protected]>

* fix: add Premium license warning to external_agent resource description

The warning needs to be in the Go code description field so it gets
auto-generated into the documentation.

Co-authored-by: M Atif Ali <[email protected]>

* docs: remove manual markdown edit to let auto-generation handle warning

The warning is now properly added in the Go code description field
and will be auto-generated into the documentation.

Co-authored-by: M Atif Ali <[email protected]>

* docs: add Premium license warning with pricing link to external_agent

Updated the Go code description to include a warning about Premium
license requirement with a link to pricing, then ran make gen to
regenerate the documentation.

Co-authored-by: M Atif Ali <[email protected]>

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Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>
Co-authored-by: M Atif Ali <[email protected]>

feat: implement default preset (#414)

feat: add `coder_ai_task` resource (#413)