feat(tools): create_project_from_template

Generates a new repo from mathias/template-go-web via Gitea's generate
API, then substitutes __PROJECT_NAME__ and __MODULE_PATH__ placeholders
in six known files (best-effort, partial failure surfaced in result).

Validates name regex, allowlist, template flag, and destination
non-existence before generating. Adds Template field to gitea.Repo.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/gitea/templates.go

@@ -0,0 +1,72 @@
+package gitea
+
+import (
+	"context"
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+// GenerateFromTemplateArgs is the request body for POST /repos/{owner}/{repo}/generate.
+type GenerateFromTemplateArgs struct {
+	Owner         string `json:"owner"`
+	Name          string `json:"name"`
+	Description   string `json:"description,omitempty"`
+	Private       bool   `json:"private"`
+	DefaultBranch string `json:"default_branch,omitempty"`
+	GitContent    bool   `json:"git_content"` // include all template files
+}
+
+// GenerateFromTemplate creates a new repository from a template via POST /repos/{tmplOwner}/{tmplName}/generate.
+func (c *Client) GenerateFromTemplate(ctx context.Context, tmplOwner, tmplName string, args GenerateFromTemplateArgs) (*Repo, error) {
+	p := fmt.Sprintf("/api/v1/repos/%s/%s/generate", tmplOwner, tmplName)
+	payload, err := json.Marshal(args)
+	if err != nil {
+		return nil, err
+	}
+	body, status, err := c.PostJSON(ctx, p, payload)
+	if err != nil {
+		return nil, err
+	}
+	if err := MapStatus(status, body); err != nil {
+		return nil, err
+	}
+	var r Repo
+	if err := json.Unmarshal(body, &r); err != nil {
+		return nil, err
+	}
+	return &r, nil
+}
+
+// SubstituteFile reads a file from the given branch, applies string replacements,
+// and writes it back if any changes were made. Best-effort — returns a wrapped error
+// that includes the file path.
+func (c *Client) SubstituteFile(ctx context.Context, owner, repo, branch, path string, replacements map[string]string) error {
+	fc, err := c.GetFileContents(ctx, owner, repo, path, branch)
+	if err != nil {
+		return fmt.Errorf("read %s: %w", path, err)
+	}
+	decoded, err := base64.StdEncoding.DecodeString(fc.Content)
+	if err != nil {
+		return fmt.Errorf("decode %s: %w", path, err)
+	}
+	content := string(decoded)
+	for k, v := range replacements {
+		content = strings.ReplaceAll(content, k, v)
+	}
+	if content == string(decoded) {
+		return nil // no changes, skip write
+	}
+	encoded := base64.StdEncoding.EncodeToString([]byte(content))
+	_, err = c.UpsertFile(ctx, owner, repo, path, UpsertFileArgs{
+		Branch:  branch,
+		Content: encoded,
+		Message: "Apply template substitutions",
+		Sha:     fc.Sha,
+	})
+	if err != nil {
+		return fmt.Errorf("write %s: %w", path, err)
+	}
+	return nil
+}