Use Trix-compatible HTML for proper paragraph spacing

[nnemirovsky]

Summary

Rebased onto current main. The original three commits targeted the pre-#415 regex-based MarkdownToHTML pipeline, which #415 replaced with goldmark. This PR is now a single commit that re-implements the same Trix-compatibility goals on the goldmark architecture.

• <div> instead of <p> — Trix stores paragraphs as <div> blocks. goldmark's default <p> output renders in Basecamp with the block margins of <p> plus the intervening <br>, producing double spacing where the user typed a single blank line.
• <div><br></div> for blank lines — Trix's native empty-line shape, replacing the bare <br>\n the TrixBreak node previously emitted.
• HTMLToMarkdown round-trip — accept <div> and <div><br></div> on input so content authored in Basecamp parses cleanly back to Markdown (and so MarkdownToHTML's own output round-trips).

Before: MarkdownToHTML("Line 1\n\nLine 2") → <p>Line 1</p>\n<br>\n<p>Line 2</p> (double-spaced in Basecamp).

After: MarkdownToHTML("Line 1\n\nLine 2") → <div>Line 1</div>\n<div><br></div>\n<div>Line 2</div> (correct single spacing).

Implementation notes

• trixRenderer now registers renderParagraph so top-level paragraphs become <div>. trixTransformer already converts paragraphs inside lists and blockquotes into TextBlock nodes, so the override only fires where it should — list/blockquote output is unchanged.
• renderTrixBreak emits <div><br></div>\n instead of <br>\n. renderHTMLBlock's raw-HTML wrapper matches.
• HTMLToMarkdown gains reDivBr and reDiv passes running before the existing <p> pipeline, so documents containing either shape (or a mix) degrade cleanly.
• blockquoteInnerToMarkdown also handles <div> / <div><br></div>, otherwise the inner <div><br></div> cascades through the outer blockquote line-splitter and produces extra > lines. TestEditLoopRoundTrip/multi-paragraph_blockquote covers this.
• Inline <br> inside a paragraph (consecutive lines joined with a single \n) is unchanged — trixTransformer's soft→hard break conversion keeps working, matching Trix's own representation.

Test plan

• All TestMarkdownToHTML* cases updated to the new <div> / <div><br></div> shape and green.
• TestEditLoopRoundTrip round-trips including multi-paragraph blockquotes.
• TestResolveMentions* inputs updated to <div> wrapper (representative of what MarkdownToHTML now emits and what Trix emits natively). Function behavior is input-shape-preserving so coverage is unchanged.
• Cross-package: TestCheckinsAnswerCreateDefaultsDateToToday updated for <div> output.
• make fmt-check, go vet, golangci-lint clean.
• go test ./... full suite green.
• End-to-end: posted a multi-paragraph comment via CLI, confirmed single-line paragraph spacing in Basecamp matches a hand-edited reference comment's HTML.

[Copilot]

Pull request overview

Updates the rich-text Markdown→HTML conversion to match Basecamp/Trix’s native HTML structure so paragraph spacing and single line breaks render consistently in Basecamp.

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Changes:

• Switch paragraph wrapper output from <p> to Trix-style <div>.
• Represent blank lines as <div><br></div> instead of emitting a standalone <br>.
• Preserve single newlines inside paragraphs by joining lines with <br> rather than spaces.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
internal/richtext/richtext.go	Emits Trix-compatible <div> blocks and <div><br></div> separators; preserves single newlines via <br>.
internal/richtext/richtext_test.go	Updates expected HTML outputs and adjusts round-trip behavior checks for the new line-break handling.
internal/commands/checkins_test.go	Updates check-in answer creation test expectation to the new <div>-wrapped content.

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[cubic-dev-ai]

1 issue found across 3 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="internal/richtext/richtext.go">

<violation number="1" location="internal/richtext/richtext.go:178">
P1: MarkdownToHTML now emits `<div>` paragraphs, but HTMLToMarkdown only normalizes `<p>` blocks, causing paragraph-spacing loss in round-trip conversions.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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