feat(pr): merge with the repository's defaults, ready, gh flags and ✓ lines for pr mutations

pr merge takes the method from the repository's default merge style
instead of always merging (D22), accepts gh's --merge, --squash and
--rebase next to -s/--method, refuses methods the repository disables,
and adds -d/--delete-branch (defaulting to the repository setting),
--auto, -t, -b, -F and -e. On a terminal it asks for the method and
branch deletion with the defaults preselected, without a picker when
the current branch has a pull request. It reports "✓ Squashed and
merged pull request alice/x#3 (title)", the deleted remote branch, and
switches off and deletes the local head branch; --json returns the
merged pull request. Merged, closed and draft pull requests are
reported before anything is asked. -s without a value points at
--squash.

pr checkout names the local branch after the head branch when that
name is free or already at the head, falling back to OWNER/BRANCH for
forks and pr/NUMBER (D13), fetches and switches quietly, reports
"✓ Checked out #1 into feat/one", and has the alias co. pr clean
reports what it deleted and that a remote branch was already gone.

pr create asks for the title and body, then "What's next?": submit,
submit as a draft, add metadata or change the branches, without the
AGit question up front or a final confirmation. It pushes an unpushed
head with --set-upstream --quiet ("✓ Pushed feat/one to origin"),
prints "✓ Created pull request alice/x#9 (title)" and the URL, and
adds --fill, --web, the alias new, the gh shorthands, and milestones
by title.

New tea pr ready [--undo]. pr close and reopen take -c/--comment,
close takes -d/--delete-branch, and both say when the pull request
already is in that state. pr edit reports "✓ Edited", takes the
singular gh reviewer flags (the plurals still work), milestones by
title and --remove-milestone. pr diff is colored and paged on a
terminal and byte-exact when piped, with --name-only, --color and
--web. Every pull request argument accepts #N, URLs and branch names
and defaults to the current branch's pull request.

Signed-off-by: bircni <bircni@icloud.com>

internal/app/errors_test.go

@@ -47,8 +47,8 @@ func TestExplainNamesResourcesAndTellsWhyRepositoriesFail(t *testing.T) {
 		"POST /api/v1/repos/alice/old/issues":             {"423", `{"message":"repo is archived"}`},
 		"POST /api/v1/repos/alice/project/pulls":          {"409", `{"message":"pull request already exists for these targets [id: 16, issue_id: 3, head_repo_id: 1, base_repo_id: 1, head_branch: feat, base_branch: main]"}`},
 		"GET /api/v1/repos/alice/project/pulls/main/feat": {"200", `{"number":3,"html_url":"https://example.org/alice/project/pulls/3"}`},
-		"GET /api/v1/repos/alice/project/pulls/3":         {"200", `{"number":3,"state":"open","draft":true,"head":{"sha":"abc"}}`},
-		"POST /api/v1/repos/alice/project/pulls/3/merge":  {"405", `{"message":"Work in progress PRs cannot be merged"}`},
+		"GET /api/v1/repos/alice/project/pulls/3":         {"200", `{"number":3,"state":"open","mergeable":false,"head":{"sha":"abc"}}`},
+		"POST /api/v1/repos/alice/project/pulls/3/merge":  {"405", `{"message":"Please try again later"}`},
 	})
 	for _, tc := range []struct {
 		name string
@@ -63,7 +63,7 @@ func TestExplainNamesResourcesAndTellsWhyRepositoriesFail(t *testing.T) {
 		{"empty repository", []string{"pr", "list", "-R", "alice/empty"}, "alice/empty is empty; it has no pull requests"},
 		{"archived repository", []string{"issue", "create", "-R", "alice/old", "--title", "x", "--body", "y"}, "alice/old is archived (read-only); unarchive it with: tea repo unarchive alice/old"},
 		{"existing pull request", []string{"pr", "create", "-R", "alice/project", "--head", "feat", "--base", "main", "--title", "x", "--body", "y"}, "pull request for feat into main already exists in alice/project; see https://example.org/alice/project/pulls/3"},
-		{"context added by the command", []string{"pr", "merge", "3", "-R", "alice/project", "--yes"}, "pull request #3 is a draft; mark it ready for review first: Gitea API returned HTTP 405 for POST /api/v1/repos/alice/project/pulls/3/merge: Work in progress PRs cannot be merged"},
+		{"context added by the command", []string{"pr", "merge", "3", "-R", "alice/project", "--yes"}, "pull request #3 has conflicting files or is otherwise not mergeable: Gitea API returned HTTP 405 for POST /api/v1/repos/alice/project/pulls/3/merge: Please try again later"},
 		{"raw api response", []string{"api", "users/nobody"}, "not found (HTTP 404 GET /api/v1/users/nobody)"},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
@@ -85,7 +85,7 @@ func TestExplainAddsTheNextStepToRefusals(t *testing.T) {
 	}{
 		{"PATCH", "/api/v1/repos/alice/x/pulls/3", "cannot change state of this pull request, it was already merged", 412, "pull request #3 is already merged; its state can no longer change"},
 		{"POST", "/api/v1/repos/alice/x/pulls/3/merge", "The PR is already merged", 405, "pull request #3 is already merged"},
-		{"POST", "/api/v1/repos/alice/x/pulls/5/merge", "Work in progress PRs cannot be merged", 405, "pull request #5 is a draft; mark it ready with: tea pr edit 5 --ready"},
+		{"POST", "/api/v1/repos/alice/x/pulls/5/merge", "Work in progress PRs cannot be merged", 405, "pull request #5 is a draft; mark it ready with: tea pr ready 5"},
 		{"POST", "/api/v1/repos/alice/x/pulls/1/reviews", "approve your own pull is not allowed", 422, "you can't approve or request changes on your own pull request; leave a comment instead"},
 		{"POST", "/api/v1/repos/alice/x/pulls", "Invalid PullRequest: There are no changes between the head and the base", 422, "no commits between the base and head branches; check out your feature branch or pass --head"},
 		{"POST", "/api/v1/repos/alice/x/actions/workflows/nightly.yaml/dispatches", "workflow is disabled", 403, "workflow nightly.yaml is disabled; enable it with: tea workflow enable nightly.yaml"},