Choose a specific organization-owned fork as the pull request head

The change involves the gh pr create command. Start by examining the command's argument parsing and the logic for resolving the head repository in the codebase, likely in the pkg/cmd/pr/create directory. Look for existing handling of the --head flag and how it interacts with forks. The feature requires understanding GitHub's API for creating pull requests and how to specify a repository-qualified head. Testing will involve verifying the new syntax works correctly and that the --web flag error is properly implemented.

Describe the feature or problem you’d like to solve

An organization can own more than one fork that could supply a pull request branch. An organization and branch alone are therefore not always enough to identify the contributor’s intended head repository.

As a contributor working in an organization with multiple forks, I want to identify the exact head repository, so that the pull request is created from the fork I intended.

Proposed solution

Allow gh pr create --head ORGANIZATION/REPOSITORY:BRANCH to identify the exact organization-owned head repository.

Feature: Choose a specific organization-owned fork as the pull request head

  Scenario: The selected fork belongs to another organization
    Given the base repository is "org/upstream"
    And "other-org/upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head other-org/upstream-fork:feature-branch`
    Then a pull request is created from "other-org/upstream-fork:feature-branch"
    And the pull request targets "org/upstream"

  Scenario: The selected fork belongs to the same organization as the base repository
    Given the base repository is "org/upstream"
    And "org/upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head org/upstream-fork:feature-branch`
    Then a pull request is created from "org/upstream-fork:feature-branch"
    And the pull request targets "org/upstream"

  Scenario: The organization owns multiple candidate forks
    Given the base repository is "org/upstream"
    And "org/upstream-fork" is a fork of the base repository
    And "org/experimental-upstream-fork" is a fork of the base repository
    When the contributor runs `gh pr create --head org/upstream-fork:feature-branch`
    Then a pull request is created from "org/upstream-fork:feature-branch"
    And the pull request targets "org/upstream"

  Scenario: Repository-qualified head is combined with web mode
    When the contributor runs `gh pr create --head other-org/upstream-fork:feature-branch --web`
    Then the command fails before opening a browser
    And the error explains that a repository-qualified `--head` cannot be used with `--web`
    And the error tells the contributor to omit `--web` or select the fork on GitHub

Additional context

This is a sub-issue of #10093.

This issue introduces the repository-qualified ORGANIZATION/REPOSITORY:BRANCH form. The --web behavior may be refined if GitHub’s web pull-request flow can identify the exact head repository.