Remove broken DCMP source causing verify-sources workflow failure

[Copilot]

The verify-sources workflow failed on a single authoritative link that now returns a server error: https://dcmp.org/learn/captioningkey in the Media Accessibility agent definition. This blocked source validation despite all other references resolving or being handled as redirects/blocked domains.

• Problem scope

  • Failure was isolated to one broken URL in .github/agents/media-accessibility.agent.md under ## Authoritative Sources.

• Change applied

  • Removed the stale DCMP Captioning Key entry that is no longer reliably reachable by the source validator.
  • No behavioral or workflow logic changes were introduced; this is a documentation-source correction to unblock validation.

• Patch illustration

  ## Authoritative Sources
  
  - **WCAG 1.2 Time-Based Media** — <https://www.w3.org/WAI/WCAG22/Understanding/time-based-media.html>
  - **W3C WebVTT Spec** — <https://www.w3.org/TR/webvtt1/>
  - **W3C Media Accessibility User Requirements** — <https://www.w3.org/TR/media-accessibility-reqs/>
  - **DCMP Captioning Key** — <https://dcmp.org/learn/captioningkey>   # removed

[Copilot]

Pull request overview

This PR removes a single broken authoritative-source URL from the Media Accessibility agent definition to unblock the verify-sources workflow, which validates external links in agent and documentation markdown.

Changes:

• Removed the stale DCMP Captioning Key source entry (https://dcmp.org/learn/captioningkey) from .github/agents/media-accessibility.agent.md.