feat(project_create): mirror_to_github opt-in, default false (infra#34 ADR)

Per the Gitea-as-true-master ADR (infra#34), GitHub mirror is now an
explicit opt-in via mirror_to_github=true. Default (omit / false) provisions
a Gitea repo + staging namespace + experiment-brief issue only — no GitHub
repo, no push-mirror.

Rationale: US cloud providers (Microsoft/GitHub) are subject to CLOUD Act
and NSL. Client code, business logic, and infra-adjacent repos should
never live on US-owned infrastructure. Only open-source projects intended
for public community (hyperguild, gitea-mcp, template-*) should opt in.

Changes
- internal/skills/project/handlers.go
  - createArgs gains MirrorToGitHub bool (json:"mirror_to_github,omitempty").
  - res.GitHubURL is set only when MirrorToGitHub is true; empty string otherwise.
  - Steps 2 (create_github_repo) + 3 (mirror) are wrapped in `if args.MirrorToGitHub`.
  - experimentBrief renders "Gitea-only" line by default and the existing
    "Push-mirror configured" line only on opt-in.
- internal/skills/project/skill.go
  - Tool schema gains mirror_to_github (boolean, default false) with description
    spelling out when to opt in. Tool Description updated to reflect new default.
- internal/skills/project/handlers_test.go
  - Added mirroredArgs() helper (happyArgs + mirror_to_github:true).
  - Tests that exercise the GitHub flow (HappyPath, GitHubExists_Idempotent,
    GitHubFails, NoGitHubClient_DegradedMode, Idempotent_RepoExists,
    MirrorFails, InfraCommitFails) switched to mirroredArgs.
  - Added TestProjectCreate_DefaultSkipsGitHubMirror covering the Gitea-only
    path: 3 gitea-mcp calls, zero GitHub calls, empty github_url, reached=
    [create_repo, infra_commit, issue], body reflects Gitea-only.

Closes gitea/mathias/hyperguild#17. Moves infra#34 acceptance item
"project_create updated: mirror_to_github defaults to false".

internal/skills/project/handlers.go

@@ -13,12 +13,13 @@ import (
 )
 
 type createArgs struct {
-	Name        string `json:"name"`
-	Description string `json:"description"`
-	Hypothesis  string `json:"hypothesis"`
-	Folder      string `json:"folder"`
-	Stack       string `json:"stack"`
-	Private     bool   `json:"private"`
+	Name           string `json:"name"`
+	Description    string `json:"description"`
+	Hypothesis     string `json:"hypothesis"`
+	Folder         string `json:"folder"`
+	Stack          string `json:"stack"`
+	Private        bool   `json:"private"`
+	MirrorToGitHub bool   `json:"mirror_to_github,omitempty"`
 }
 
 type createResult struct {
@@ -59,11 +60,12 @@ func (s *Skill) handleCreate(ctx context.Context, raw json.RawMessage) (json.Raw
 
 	tmpl := templateFor(args.Stack)
 	giteaURL := fmt.Sprintf("http://gitea.d-ma.be/%s/%s", s.cfg.GiteaOwner, args.Name)
-	githubURL := fmt.Sprintf("https://github.com/%s/%s", s.cfg.GitHubOwner, args.Name)
 
 	res := createResult{
-		GiteaURL:  giteaURL,
-		GitHubURL: githubURL,
+		GiteaURL: giteaURL,
+	}
+	if args.MirrorToGitHub {
+		res.GitHubURL = fmt.Sprintf("https://github.com/%s/%s", s.cfg.GitHubOwner, args.Name)
 	}
 
 	// Step 1: create_project_from_template. If the repo already exists,
@@ -75,25 +77,32 @@ func (s *Skill) handleCreate(ctx context.Context, raw json.RawMessage) (json.Raw
 	}
 	res.Reached = append(res.Reached, stepCreateRepo)
 
-	// Step 2: create empty GitHub repo. Gitea's push-mirror cannot push
-	// to a non-existent remote, so the destination must exist before
-	// step 3 configures the mirror. Skipped when GitHub client is unset
-	// (degraded mode — see Config.GitHub doc).
-	if s.cfg.GitHub != nil {
-		if err := s.callCreateGitHubRepo(ctx, args); err != nil && !errors.Is(err, githubclient.ErrAlreadyExists) {
-			return marshalPartial(res, stepCreateGitHub, err)
+	// Steps 2+3 are skipped when MirrorToGitHub is false. Default per
+	// infra ADR (Gitea as true master, GitHub as optional opt-in): keep
+	// client / business-logic / personal repos Gitea-only. Set
+	// `mirror_to_github: true` for open-source projects that want a
+	// public GitHub mirror (hyperguild, gitea-mcp, template-*).
+	if args.MirrorToGitHub {
+		// Step 2: create empty GitHub repo. Gitea's push-mirror cannot push
+		// to a non-existent remote, so the destination must exist before
+		// step 3 configures the mirror. Skipped when GitHub client is unset
+		// (degraded mode — see Config.GitHub doc).
+		if s.cfg.GitHub != nil {
+			if err := s.callCreateGitHubRepo(ctx, args); err != nil && !errors.Is(err, githubclient.ErrAlreadyExists) {
+				return marshalPartial(res, stepCreateGitHub, err)
+			}
+			res.Reached = append(res.Reached, stepCreateGitHub)
 		}
-		res.Reached = append(res.Reached, stepCreateGitHub)
-	}
 
-	// Step 3: configure push mirror to GitHub. Idempotent: if a mirror with
-	// the same remote already exists, gitea-mcp returns Conflict; we swallow it.
-	if err := s.callMirror(ctx, args.Name); err != nil {
-		if !isConflict(err) {
-			return marshalPartial(res, stepMirror, err)
+		// Step 3: configure push mirror to GitHub. Idempotent: if a mirror with
+		// the same remote already exists, gitea-mcp returns Conflict; we swallow it.
+		if err := s.callMirror(ctx, args.Name); err != nil {
+			if !isConflict(err) {
+				return marshalPartial(res, stepMirror, err)
+			}
 		}
+		res.Reached = append(res.Reached, stepMirror)
 	}
-	res.Reached = append(res.Reached, stepMirror)
 
 	// Step 3: commit staging namespace manifest to infra repo. Done before
 	// the issue so the staging env is reconciling by the time the issue lands.
@@ -228,7 +237,11 @@ func experimentBrief(args createArgs, existed bool) string {
 	b.WriteString("- Repo created from `template-")
 	b.WriteString(args.Stack)
 	b.WriteString("` on Gitea.\n")
-	b.WriteString("- Push-mirror configured to GitHub.\n")
+	if args.MirrorToGitHub {
+		b.WriteString("- Push-mirror configured to GitHub.\n")
+	} else {
+		b.WriteString("- Gitea-only (no GitHub mirror — set `mirror_to_github: true` to opt in).\n")
+	}
 	b.WriteString("- Staging namespace manifest committed to infra repo.\n\n")
 	if existed {
 		b.WriteString("> Note: this repo already existed when `project_create` ran — provisioning steps were re-applied idempotently.\n")