Render GFM tables in message and comment bodies (#405)

[jeremy]

Fixes #405.

GFM Markdown tables in message/comment bodies rendered as raw pipe text instead of <table> HTML — every other Markdown construct converted, tables were the exception. mdConverter registered only extension.Strikethrough, so pipe rows parsed as plain paragraphs.

Changes

• Outbound: register goldmark's table extension with the align-attribute cell method — extension.NewTable(WithTableCellAlignMethod(TableCellAlignAttribute)). This emits the whitelisted align attribute rather than the default style="text-align:…", which BC3's sanitizer strips (only color/background-color CSS survive; the align attribute is whitelisted — verified in bc3 app/helpers/content_pipeline/sanitization.rb). So GFM column alignment survives sanitization. Bare <table> is correct — BC3's WrapTablesFilter supplies the <figure class="lexxy-content__table-wrapper"> wrapper.
• Chat detection: IsMarkdown gains AST-based table detection (parses with the table extension, walks for east.KindTable) so a table-only chat message is sent as HTML, not raw pipes.
• Display: IsHTML recognizes <table> so a plain BC3 table response routes through the display converter instead of leaking raw markup to the reader.
• Fail-closed edit guards: new richtext.HasTableHTML; the TUI in-place editors (message body, comment, to-do description) refuse to open table-bearing content rather than flatten it — HTMLToMarkdown has no table handling, so entering edit mode and resubmitting would strip the table. Round-trip editing waits on server-side Markdown (BC3 #11986).
• Docs: the Basecamp skill lists tables as supported and documents the edit boundary.

Relationship to #498

@savtrip's #498 makes the one-line extension registration. This PR carries the align-attribute correction (so column alignment isn't lost to the sanitizer) plus the surrounding work that keeps the CLI from destroying tables now that it can author them. The align-attribute fix was also offered to #498 as an inline suggestion; this PR is the home for the broader AST-detection + display-routing + edit-guard + tests changes that don't fit an inline suggestion.

Testing

bin/ci fully green (fmt, vet, lint, unit, e2e, surface, skill drift, provenance, mod tidy). New tests cover the #405 golden repro, column alignment, IsMarkdown/IsHTML/HasTableHTML table cases, the composer table-only regression, and the fail-closed edit guards.

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Summary by cubic

Render GFM tables in message and comment bodies as real HTML tables and preserve column alignment through BC3 sanitization. Also detect and display tables correctly, and block in‑place edits that would strip them. Fixes #405.

• Bug Fixes
  • Register goldmark table extension using align attributes instead of inline styles; emit bare <table> (BC3 wraps it).
  • Composer: AST-based table detection so table-only posts are sent as HTML, not raw pipes.
  • Display: IsHTML now recognizes <table> and routes BC3 table responses through the display converter.
  • TUI editors: refuse to open message/comment/to‑do description that contains a table to avoid losing structure; docs updated to note the boundary.

^{Written for commit 601b272. Summary will update on new commits.}

[Copilot]

Pull request overview

This PR fixes #405 by enabling GitHub Flavored Markdown (GFM) table rendering in message/comment bodies, ensuring table-only content is correctly detected as Markdown/HTML, and preventing the TUI from “round-tripping” table content through HTML→Markdown in-place editors in a way that would silently destroy table structure.

Changes:

• Register Goldmark’s table extension (configured to emit align attributes) so Markdown tables render as <table> HTML and preserve column alignment through BC3 sanitization.
• Improve content-type heuristics: detect Markdown tables via AST parsing and treat <table> as HTML for display routing.
• Add fail-closed TUI edit guards (with tests) that block in-place editing when HTML contains tables, avoiding destructive re-submits; document the boundary in the Basecamp skill.

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Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
skills/basecamp/SKILL.md	Documents table support and the TUI edit boundary for table-bearing content.
internal/tui/workspace/widget/composer_test.go	Adds coverage ensuring table-only submissions aren’t misclassified as plain text.
internal/tui/workspace/views/todos.go	Blocks in-place editing of to-do descriptions containing HTML tables.
internal/tui/workspace/views/todos_test.go	Tests the fail-closed edit guard for table-bearing descriptions.
internal/tui/workspace/views/detail.go	Blocks in-place editing for message bodies and comments containing HTML tables.
internal/tui/workspace/views/detail_test.go	Tests table edit guards for both message bodies and comments.
internal/richtext/richtext.go	Adds table rendering + table-aware Markdown/HTML detection and introduces HasTableHTML.
internal/richtext/richtext_test.go	Adds regression tests for table rendering/alignment and detection helpers.

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[jeremy]

Closing — this work belongs on #498, not a separate PR. The align-attribute fix is already offered there as an inline suggestion; reconciling the rest onto #498.