feat(send): auto-convert Markdown to Slack mrkdwn with --blocks and --raw flags

Messages now auto-convert standard Markdown (**bold**, # headers, [links](url))
to Slack mrkdwn (*bold*, etc). Formatting hints shown on stderr when conversions
are applied. --blocks sends via Block Kit sections for richer rendering with
automatic splitting at Slack's 3000-char limit. --raw disables auto-conversion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: isaacrowntree

internal/text/mrkdwn.go

@@ -0,0 +1,161 @@
+package text
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+// Slack mrkdwn reference:
+//   *bold*    _italic_    ~strikethrough~    `code`    ```code block```
+//   > blockquote    • or - for bullets (- works natively)
+//   <url|label> for links    :emoji: shortcodes
+//   No headers (#), no **bold**, no [links](url), no ![images](url)
+
+var (
+	// Markdown bold: **text** or __text__ → *text*
+	mdBoldDoubleAsterisk = regexp.MustCompile(`\*\*(.+?)\*\*`)
+	mdBoldDoubleUnderscore = regexp.MustCompile(`__(.+?)__`)
+
+	// Markdown italic: *text* is already valid mrkdwn, but _text_ also works.
+	// We only need to handle cases where markdown uses single * for italic
+	// and the user also has **bold** — the bold conversion handles that.
+
+	// Markdown headers: # Header, ## Header, ### Header
+	// Convert to *bold text* on its own line
+	mdHeader = regexp.MustCompile(`(?m)^(#{1,6})\s+(.+)$`)
+
+	// Markdown links: [text](url) → <url|text>
+	mdLink = regexp.MustCompile(`\[([^\]]+)\]\(([^)]+)\)`)
+
+	// Markdown images: ![alt](url) → <url|alt> (best effort)
+	mdImage = regexp.MustCompile(`!\[([^\]]*)\]\(([^)]+)\)`)
+
+	// Markdown horizontal rules: --- or *** or ___ on their own line
+	// Convert to ───── divider (Slack has no native HR in mrkdwn text)
+	mdHorizontalRule = regexp.MustCompile(`(?m)^[\s]*([-*_]){3,}\s*$`)
+
+	// Markdown ordered list: 1. item → 1. item (already works, but detect for hints)
+	mdOrderedList = regexp.MustCompile(`(?m)^\s*\d+\.\s+`)
+
+	// Detect markdown-style bold that won't render in Slack
+	detectDoubleBold = regexp.MustCompile(`\*\*[^*]+\*\*`)
+
+	// Detect markdown headers
+	detectHeaders = regexp.MustCompile(`(?m)^#{1,6}\s+`)
+
+	// Detect markdown links
+	detectLinks = regexp.MustCompile(`\[[^\]]+\]\([^)]+\)`)
+
+	// Detect markdown images
+	detectImages = regexp.MustCompile(`!\[[^\]]*\]\([^)]+\)`)
+)
+
+// FormatHint describes a formatting issue detected in the message text.
+type FormatHint struct {
+	Issue      string // What was detected
+	Suggestion string // How to fix it for Slack
+	Example    string // Before → After example
+}
+
+// ConvertMarkdownToMrkdwn converts common Markdown syntax to Slack mrkdwn.
+// It handles bold, headers, links, images, and horizontal rules.
+// Already-valid mrkdwn (like *bold*, _italic_, ~strike~, `code`) passes through unchanged.
+func ConvertMarkdownToMrkdwn(text string) string {
+	// Order matters: images before links (images contain link syntax)
+
+	// Images: ![alt](url) → <url|alt> or just <url>
+	text = mdImage.ReplaceAllStringFunc(text, func(match string) string {
+		parts := mdImage.FindStringSubmatch(match)
+		alt, url := parts[1], parts[2]
+		if alt != "" {
+			return fmt.Sprintf("<%s|%s>", url, alt)
+		}
+		return fmt.Sprintf("<%s>", url)
+	})
+
+	// Links: [text](url) → <url|text>
+	text = mdLink.ReplaceAllStringFunc(text, func(match string) string {
+		parts := mdLink.FindStringSubmatch(match)
+		label, url := parts[1], parts[2]
+		return fmt.Sprintf("<%s|%s>", url, label)
+	})
+
+	// Headers first: # Text → *Text* (before bold, so nested **bold** in headers
+	// gets cleaned up by the bold pass)
+	text = mdHeader.ReplaceAllStringFunc(text, func(match string) string {
+		parts := mdHeader.FindStringSubmatch(match)
+		// Strip any ** markers inside the header text before wrapping in *
+		inner := mdBoldDoubleAsterisk.ReplaceAllString(parts[2], "$1")
+		inner = mdBoldDoubleUnderscore.ReplaceAllString(inner, "$1")
+		return "*" + inner + "*"
+	})
+
+	// Bold: **text** → *text*
+	text = mdBoldDoubleAsterisk.ReplaceAllString(text, "*$1*")
+	text = mdBoldDoubleUnderscore.ReplaceAllString(text, "*$1*")
+
+	// Horizontal rules: --- → ─────
+	text = mdHorizontalRule.ReplaceAllString(text, "─────")
+
+	return text
+}
+
+// DetectFormatHints inspects message text and returns hints about Markdown
+// syntax that won't render correctly in Slack. Call this BEFORE conversion
+// to give the user actionable feedback.
+func DetectFormatHints(text string) []FormatHint {
+	var hints []FormatHint
+
+	if detectDoubleBold.MatchString(text) {
+		hints = append(hints, FormatHint{
+			Issue:      "Markdown bold (**text**) won't render in Slack",
+			Suggestion: "Use *text* for bold in Slack mrkdwn",
+			Example:    "**bold** → *bold*",
+		})
+	}
+
+	if detectHeaders.MatchString(text) {
+		hints = append(hints, FormatHint{
+			Issue:      "Markdown headers (# Header) aren't supported in Slack",
+			Suggestion: "Use *bold text* on its own line for emphasis",
+			Example:    "# Section → *Section*",
+		})
+	}
+
+	if detectLinks.MatchString(text) {
+		hints = append(hints, FormatHint{
+			Issue:      "Markdown links [text](url) won't render in Slack",
+			Suggestion: "Use <url|text> for Slack links",
+			Example:    "[Click here](https://...) → <https://...|Click here>",
+		})
+	}
+
+	if detectImages.MatchString(text) {
+		hints = append(hints, FormatHint{
+			Issue:      "Markdown images ![alt](url) aren't supported in Slack",
+			Suggestion: "Use <url|alt> — Slack will unfurl image URLs automatically",
+			Example:    "![logo](https://...) → <https://...|logo>",
+		})
+	}
+
+	return hints
+}
+
+// SlackMrkdwnReference returns a quick-reference string for Slack mrkdwn syntax.
+func SlackMrkdwnReference() string {
+	return strings.TrimSpace(`
+Slack mrkdwn formatting reference:
+  *bold*           Bold text
+  _italic_         Italic text
+  ~strikethrough~  Strikethrough
+  ` + "`code`" + `          Inline code
+  ` + "```code```" + `      Code block
+  > quote          Blockquote
+  • item           Bullet list (or - item)
+  1. item          Numbered list
+  <url|label>      Hyperlink
+  :emoji:          Emoji shortcode
+  @user            Mention (auto-resolved by slackbuzz)
+`)
+}