feat: surface template README to agent template tools (#26334)

Fixes CODAGT-447.

Alternative implementation of https://github.com/coder/coder/pull/26212
and https://github.com/coder/coder/pull/25978

- Adds up to the first 1000 characters of `README.md` (with leading
frontmatter stripped) to `chattool.list_templates` output
- Adds up to 800 characters of `README.md` to `chattool.read_template`.

**Note:** skipping `toolsdk` versions to keep scope small.

> 🤖 Generated by Coder Agents

coderd/render/markdown.go

@@ -9,9 +9,22 @@ import (
 	gomarkdown "github.com/gomarkdown/markdown"
 	"github.com/gomarkdown/markdown/html"
 	"github.com/gomarkdown/markdown/parser"
+	"github.com/yuin/goldmark"
+	"github.com/yuin/goldmark/extension"
+	goldmarkhtml "github.com/yuin/goldmark/renderer/html"
+	xhtml "golang.org/x/net/html"
 	"golang.org/x/xerrors"
 )
 
+// innerTextMarkdown converts Markdown to HTML for InnerTextFromMarkdown. Table
+// renders cells as text (not pipe-delimited lines); WithUnsafe lets embedded raw
+// HTML through so its inner text survives. Safe to share: goldmark inits the
+// parser once via sync.Once, then only reads it.
+var innerTextMarkdown = goldmark.New(
+	goldmark.WithExtensions(extension.Table),
+	goldmark.WithRendererOptions(goldmarkhtml.WithUnsafe()),
+)
+
 var plaintextStyle = ansi.StyleConfig{
 	Document: ansi.StyleBlock{
 		StylePrimitive: ansi.StylePrimitive{},
@@ -108,3 +121,56 @@ func HTMLFromMarkdown(markdown string) string {
 	})
 	return string(bytes.TrimSpace(gomarkdown.Render(doc, renderer)))
 }
+
+// InnerTextFromMarkdown renders Markdown (including embedded raw HTML) to HTML
+// and returns its visible text ("innerText"). Block, code-line, and table-cell
+// boundaries become newlines and intra-line whitespace is collapsed; link text
+// is kept but URLs, images, and badges are dropped.
+//
+// Input is untrusted: a parser panic is recovered and returned as an error.
+func InnerTextFromMarkdown(markdown string) (out string, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			out, err = "", xerrors.Errorf("render markdown to innertext: %v", r)
+		}
+	}()
+
+	var rendered bytes.Buffer
+	if convErr := innerTextMarkdown.Convert([]byte(markdown), &rendered); convErr != nil {
+		return "", xerrors.Errorf("convert markdown to html: %w", convErr)
+	}
+
+	z := xhtml.NewTokenizer(&rendered)
+	var b strings.Builder
+	// script and style are raw-text elements: their body is the single text token
+	// after the start tag. Skip just that token (not a running depth) so a stray
+	// </script> or unterminated tag can't swallow the rest of the document.
+	skipNextText := false
+	for {
+		if z.Next() == xhtml.ErrorToken {
+			break // includes io.EOF
+		}
+		switch tok := z.Token(); tok.Type {
+		case xhtml.StartTagToken:
+			skipNextText = tok.Data == "script" || tok.Data == "style"
+		case xhtml.TextToken:
+			if skipNextText {
+				skipNextText = false
+				continue
+			}
+			_, _ = b.WriteString(tok.Data)
+		default:
+			skipNextText = false
+		}
+	}
+
+	// Collapse intra-line whitespace but keep newlines so code lines, table
+	// cells, and block boundaries stay on separate lines; drop blank lines.
+	var lines []string
+	for _, line := range strings.Split(b.String(), "\n") {
+		if f := strings.Join(strings.Fields(line), " "); f != "" {
+			lines = append(lines, f)
+		}
+	}
+	return strings.Join(lines, "\n"), nil
+}