docs: add opencode-datadog-monitor to ecosystem

[mormubis]

Adds opencode-datadog-monitor to the ecosystem plugins list.

An OpenCode plugin that sends session, tool, and LLM traces to Datadog LLM Observability via the HTTP API. No Datadog Agent or dd-trace dependency required.

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

I found a potential duplicate:

Related PR:

• PR docs(ecosystem): add @datadog/opencode-plugin #25756: docs(ecosystem): add @datadog/opencode-plugin - docs(ecosystem): add @datadog/opencode-plugin #25756

Why they might be related:
Both PRs are adding Datadog-related plugins to the ecosystem documentation. PR #25756 appears to be for @datadog/opencode-plugin, while PR #29228 (current) is for opencode-datadog-monitor. These could be different versions, forks, or renamed versions of the same plugin. You may want to verify which one is the canonical/current plugin that should be documented.