Render nested Action Text attachment content

[nitrnitr]

What changed

Handle bare <action-text-attachment> elements whose HTML is stored in a content attribute.

HEY can nest this element inside a Trix HTML attachment. Previously, ToMarkdown treated it as a file attachment and rendered 📎 attachment, dropping the embedded text and links.

Unnamed elements with non-empty content are now rendered through the existing bounded embedded-HTML path. Named elements remain file attachments.

Added regression coverage for both the nested HTML case and filename precedence.

Testing

• make check
• make build

[Copilot]

Pull request overview

Renders unnamed nested Action Text attachment HTML through the existing bounded Markdown path while preserving named file attachments.

Changes:

• Added nested attachment-content rendering.
• Added regression and filename-precedence tests.

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Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
internal/htmlutil/markdown.go	Handles embedded Action Text attachment content.
internal/htmlutil/markdown_test.go	Tests nested HTML and filename precedence.

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[jeremy]

Rebased onto main as 6de1d33 so this applies cleanly again — #387 (decorative-image skipping) landed in the same three spots. The merge keeps #387's decorative check first in walkNode, findImages and the Markdown path, then the embedded-content branch from this PR, then the named-attachment fallback; renderActionTextAttachment is folded into main's actionTextAttachment rather than kept beside it. No behaviour change to what this PR does: an unnamed content attachment is rendered as embedded HTML, a named one stays a file attachment, and all five new tests pass as written. make check is green locally; the test workflow has not run on this PR yet (first-time-contributor approval), so the local run is the gate for now.