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
+}

coderd/render/markdown_test.go

@@ -87,3 +87,59 @@ func TestHTML(t *testing.T) {
 		})
 	}
 }
+
+func TestInnerTextFromMarkdown(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{"LinkTextKeptUrlDropped", "Use [Coder](https://coder.com/docs) now.", "Use Coder now."},
+		{"ImageDropped", "# T\n\n![alt](a.svg)\n\nBody.", "T\nBody."},
+		{"BadgeDropped", "[![discord](shield.png)](https://discord.gg/x)\n\nReal.", "Real."},
+		{"CodeBlockLinesKept", "Intro.\n\n```sh\nnpm install\nnpm run dev\n```\n\nOutro.", "Intro.\nnpm install\nnpm run dev\nOutro."},
+		{"TableCellsKept", "Before.\n\n| env | required |\n|---|---|\n| FOO | yes |\n\nAfter.", "Before.\nenv\nrequired\nFOO\nyes\nAfter."},
+		{"HtmlInnerTextKept", "<p>Important: needs GPU.</p>", "Important: needs GPU."},
+		{
+			// Markdown nested inside a block-level HTML wrapper must still be
+			// parsed (CommonMark terminates the HTML block at the blank line):
+			// nav links collapse to text, badges drop. Regresses the gomarkdown
+			// behavior that leaked raw badge markdown with URLs.
+			"MarkdownInsideHtmlBlock",
+			"<div align=\"center\">\n  <img src=\"logo.png\" alt=\"Logo\">\n</div>\n\n" +
+				"[Docs](https://x.com/docs) | [Why](https://x.com/why)\n\n" +
+				"[![badge](https://img.shields.io/x.svg)](https://x.com)\n\nReal prose.",
+			"Docs | Why\nReal prose.",
+		},
+		{"ScriptDropped", "Before.\n\n<script>alert('x')</script>\n\nAfter.", "Before.\nAfter."},
+		// An empty-body <script src=...> must not leave the skip armed and eat the
+		// next text run (guards the skipNextText reset on non-text tokens).
+		{"ScriptSrcEmptyBody", "Before.\n\n<script src=\"x.js\"></script>\n\nAfter.", "Before.\nAfter."},
+		{"StyleDropped", "Before.\n\n<style>.x{color:red}</style>\n\nAfter.", "Before.\nAfter."},
+		// A bare </script> in prose must not underflow the skip and swallow what
+		// follows.
+		{"BareScriptCloseNoUnderflow", "Before.\n\n</script>\n\nAfter.", "Before.\nAfter."},
+		// An unterminated raw-text element is, per the HTML spec, a single run to
+		// EOF, so the remainder is unavoidably consumed; it must not error.
+		{"UnterminatedScriptEatsRest", "Intro.\n\n<script>\nvar x = 1;\n\nMore prose.", "Intro."},
+		{"EmphasisAndCodeSpanFlattened", "Run `make` for **speed**.", "Run make for speed."},
+		{"HeadingParagraphOrder", "# Title\n\nLead.\n\n## Prereq\n\nDetail.", "Title\nLead.\nPrereq\nDetail."},
+		// Straight ASCII punctuation must stay ASCII (goldmark applies no
+		// Typographer), and existing smart punctuation passes through unchanged.
+		// The smart characters are \u-escaped so the docs linter does not rewrite
+		// them back to ASCII in source.
+		{"PunctuationNotRewritten", "Range 10\u201420, \"q\", ... and smart \u201cq\u201d \u2014 \u2026", "Range 10\u201420, \"q\", ... and smart \u201cq\u201d \u2014 \u2026"},
+		{"EmptyReturnsEmpty", "", ""},
+		{"WhitespaceReturnsEmpty", "   \n\t\n", ""},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			got, err := render.InnerTextFromMarkdown(tt.input)
+			require.NoError(t, err)
+			require.Equal(t, tt.expected, got)
+		})
+	}
+}