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/args_test.go

@@ -88,7 +88,7 @@ func TestArgumentErrorsShowTheUsageLine(t *testing.T) {
 		{[]string{"issue", "view"}, "missing issue number\nUsage: tea issue view <number> [flags]\nRun 'tea issue view --help' for more information."},
 		{[]string{"issue", "view", "1", "2"}, "too many arguments\nUsage: tea issue view <number> [flags]"},
 		{[]string{"issue", "list", "extra"}, "unexpected argument \"extra\"\nUsage: tea issue list [flags]"},
-		{[]string{"pr", "checkout"}, "missing pull request number\nUsage: tea pr checkout <number> [flags]"},
+		{[]string{"pr", "checkout"}, "missing pull request number\nUsage: tea pr checkout {<number> | <url> | <branch>} [flags]"},
 		{[]string{"run", "view"}, "missing run ID\nUsage: tea run view [<run-id>] [flags]"},
 		{[]string{"release", "asset", "edit", "v1"}, "missing asset\nUsage: tea release asset edit <tag> <asset> [flags]"},
 		{[]string{"repo", "clone"}, "missing repository\nUsage: tea repo clone <repository> [<directory>] [-- <gitflags>...] [flags]"},