Decouple metadata prompt contract from overridable style

Metadata prompts (workspace title + branch, commit message, PR) mixed the
functional contract with style guidance in one block, and project-owner
instructions from paseo.json were appended after the defaults with an
"override where they conflict" notice — so custom wording competed with the
built-in style instead of replacing it.

Each prompt is now a non-overridable contract (what to produce, the JSON
shape, correctness/safety rules) plus style slots that paseo.json
instructions replace wholesale. Title and branch style are now separate
keys (metadataGeneration.title vs .branchName); previously branchName
instructions also shaped the title.

packages/protocol/src/paseo-config-schema.test.ts

@@ -37,6 +37,7 @@ describe("paseo config schema", () => {
     expect(
       PaseoConfigSchema.parse({
         metadataGeneration: {
+          title: { instructions: "Keep titles to a few words." },
           branchName: { instructions: "Prefix branches with feat/." },
           commitMessage: { instructions: "Use imperative mood." },
           pullRequest: { instructions: "Include risk notes." },
@@ -44,6 +45,7 @@ describe("paseo config schema", () => {
       }),
     ).toEqual({
       metadataGeneration: {
+        title: { instructions: "Keep titles to a few words." },
         branchName: { instructions: "Prefix branches with feat/." },
         commitMessage: { instructions: "Use imperative mood." },
         pullRequest: { instructions: "Include risk notes." },

packages/protocol/src/paseo-config-schema.ts

@@ -39,6 +39,7 @@ export const PaseoMetadataGenerationEntrySchema = z
 
 export const PaseoMetadataGenerationSchema = z
   .object({
+    title: PaseoMetadataGenerationEntrySchema.optional(),
     branchName: PaseoMetadataGenerationEntrySchema.optional(),
     commitMessage: PaseoMetadataGenerationEntrySchema.optional(),
     pullRequest: PaseoMetadataGenerationEntrySchema.optional(),