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/harness/echo_test.go

@@ -9,9 +9,13 @@ import (
 )
 
 // echoSkill is the trivial test skill: it asks the model to repeat a word.
+// Its prompt is prompts/echo.v1.tmpl.
 type echoSkill struct{}
 
-type echoInput struct{ Word string }
+type echoInput struct {
+	Word    string
+	Context []ContextSection
+}
 
 type echoOutput struct {
 	Echo string `json:"echo"`
@@ -23,11 +27,11 @@ func (echoSkill) Name() string { return "echo" }
 
 func (echoSkill) Version() int { return 1 }
 
-func (echoSkill) Render(in echoInput) (Prompt, error) {
+func (echoSkill) PromptInput(in echoInput) (PromptInput, error) {
 	if in.Word == "" {
-		return Prompt{}, errors.New("no word to echo")
+		return PromptInput{}, errors.New("no word to echo")
 	}
-	return Prompt{System: "You repeat words.", User: "Repeat: " + in.Word}, nil
+	return PromptInput{Data: in, Context: in.Context}, nil
 }
 
 func (echoSkill) Schema() llm.JSONSchema {