Slice 10: create_pr (open a pull request)

forge CreatePullRequest (Gitea; empty base resolves to the repo default branch). Service CreatePR records a pr-created event. HTTP POST /api/repo/pr/create; MCP tool create_pr; <pr-list> New pull request button (head = selected repo current branch, base = default). Live-tested end-to-end: app create_pr opened a real PR (base auto-resolved), listed, then cleaned up; main untouched.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/forge/forge.go

@@ -48,11 +48,21 @@ type MergeResult struct {
 	BranchDeleted bool   `json:"branchDeleted"`
 }
 
+// NewPR describes a pull request to open. An empty Base means "the repo's
+// default branch".
+type NewPR struct {
+	Head  string
+	Base  string
+	Title string
+	Body  string
+}
+
 // Provider talks to one hosting provider.
 type Provider interface {
 	// Handles reports whether this provider serves the given remote host.
 	Handles(host string) bool
 	ListPullRequests(ctx context.Context, owner, repo string) ([]PullRequest, error)
+	CreatePullRequest(ctx context.Context, owner, repo string, pr NewPR) (PullRequest, error)
 	MergeAndCleanup(ctx context.Context, owner, repo string, number int64, method MergeMethod) (MergeResult, error)
 }