fix(pr): keep targeting the base repository after a fork PR checkout

Repository discovery preferred the current branch's upstream remote,
which for a fork pull request is the pulls/<owner>/<repo> remote that
pr checkout adds, so every later command targeted the fork. pr checkout
now records the pull request number and base remote in
branch.<name>.tea-pr and branch.<name>.tea-base-remote, discovery
honors the recorded base remote, and it skips pulls/* head remotes.

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

internal/git/checkout.go

@@ -7,9 +7,16 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strconv"
 	"strings"
 )
 
+// Branch configuration keys that record the pull request and base remote of a checked-out PR branch.
+const (
+	pullNumberKey     = "tea-pr"
+	pullBaseRemoteKey = "tea-base-remote"
+)
+
 // CheckoutOptions identifies the base remote and how to check out the PR commit.
 type CheckoutOptions struct {
 	// CredentialArgs contains per-operation Git configuration for the fetch only.
@@ -152,5 +159,12 @@ func CheckoutPull(ctx context.Context, runner Runner, options CheckoutOptions) (
 			return CheckoutResult{}, fmt.Errorf("checked out the branch, but could not configure tracking: %w", err)
 		}
 	}
+	if !options.Detached {
+		for _, setting := range [][2]string{{pullNumberKey, strconv.FormatInt(options.Number, 10)}, {pullBaseRemoteKey, options.Remote}} {
+			if _, err := run("config", "branch."+options.Branch+"."+setting[0], setting[1]); err != nil {
+				return CheckoutResult{}, fmt.Errorf("checked out the branch, but could not record its pull request: %w", err)
+			}
+		}
+	}
 	return CheckoutResult{Branch: options.Branch, Commit: commit, Detached: options.Detached, Remote: fetchRemote, Tracking: tracking}, nil
 }