Layered prompt rendering from embedded templates

Skill prompts are text/template files embedded from
internal/ai/skills/prompts/, one per skill version
(<skill>.v<version>.tmpl) defining "instructions" and "user". Templates
are parsed at start with missingkey=error; a bad template stops the
service.

harness.Render builds the system prompt in fixed order (D-007): [1] the
admin persona from ai_settings.system_prompt (Config.Persona), [2] the
skill instructions, [3] the household context block, [4] the output
contract quoting the JSON Schema. The contract is always the last layer
whatever the persona says. The household context is a seam of titled
sections until BuildContext lands; an empty one renders a defined block.
Skill.Render is replaced by Skill.PromptInput.

Golden tests pin the exact system prompt and user message for the echo
test skill, with and without persona, and under a hostile persona that
says "ignore the format below" or fakes a contract heading.
checksums.txt records each template's SHA-256; a template change without
a version bump fails TestTemplateChecksums naming the skill and version.

Maestro task bceca7a8-bc30-48df-944d-84557a1f4154.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ai/skills/prompts/prompts.go

@@ -0,0 +1,102 @@
+// Package prompts holds the skills' prompt templates, embedded from this
+// directory. Each skill version has one text/template file named
+// <skill>.v<version>.tmpl that defines two templates: "instructions", the
+// skill's layer of the system prompt, and "user", the user message. Both
+// execute with the data the skill derives from its input.
+//
+// A template never changes under a released version: checksums.txt records
+// the SHA-256 of every template, and a test fails when a template no longer
+// matches it. Changing a prompt means bumping the skill's version, which
+// renames its template file.
+package prompts
+
+import (
+	"bytes"
+	"embed"
+	"fmt"
+	"io/fs"
+	"regexp"
+	"strconv"
+	"strings"
+	"text/template"
+)
+
+//go:embed *.tmpl
+var files embed.FS
+
+// fileName matches a template file name and captures the skill name and
+// version.
+var fileName = regexp.MustCompile(`^([a-z][a-z0-9_]*)\.v([1-9][0-9]*)\.tmpl$`)
+
+// templates maps a template file name to its parsed template. A bad
+// template stops the service at start.
+var templates = mustParse(files)
+
+func mustParse(fsys fs.FS) map[string]*template.Template {
+	t, err := parse(fsys)
+	if err != nil {
+		panic(err)
+	}
+	return t
+}
+
+// parse parses every template in fsys, each on its own so that every skill
+// can define "instructions" and "user".
+func parse(fsys fs.FS) (map[string]*template.Template, error) {
+	names, err := fs.Glob(fsys, "*.tmpl")
+	if err != nil {
+		return nil, err
+	}
+	out := make(map[string]*template.Template, len(names))
+	for _, name := range names {
+		if !fileName.MatchString(name) {
+			return nil, fmt.Errorf("prompts: %s: file name is not <skill>.v<version>.tmpl", name)
+		}
+		t, err := template.New(name).Option("missingkey=error").ParseFS(fsys, name)
+		if err != nil {
+			return nil, fmt.Errorf("prompts: %w", err)
+		}
+		for _, part := range []string{"instructions", "user"} {
+			if t.Lookup(part) == nil {
+				return nil, fmt.Errorf("prompts: %s: does not define %q", name, part)
+			}
+		}
+		out[name] = t
+	}
+	return out, nil
+}
+
+// File is the name of the template file for version of skill.
+func File(skill string, version int) string {
+	return skill + ".v" + strconv.Itoa(version) + ".tmpl"
+}
+
+// Render executes the template for version of skill with data and returns
+// its instructions and user message, trimmed of surrounding white space.
+// Either being empty is an error.
+func Render(skill string, version int, data any) (instructions, user string, err error) {
+	name := File(skill, version)
+	t, ok := templates[name]
+	if !ok {
+		return "", "", fmt.Errorf("prompts: no template %s for skill %s v%d", name, skill, version)
+	}
+	if instructions, err = execute(t, "instructions", data); err != nil {
+		return "", "", err
+	}
+	if user, err = execute(t, "user", data); err != nil {
+		return "", "", err
+	}
+	return instructions, user, nil
+}
+
+func execute(t *template.Template, part string, data any) (string, error) {
+	var b bytes.Buffer
+	if err := t.ExecuteTemplate(&b, part, data); err != nil {
+		return "", fmt.Errorf("prompts: %w", err)
+	}
+	s := strings.TrimSpace(b.String())
+	if s == "" {
+		return "", fmt.Errorf("prompts: %s: %q rendered empty", t.Name(), part)
+	}
+	return s, nil
+}