feat: allow modern commands to opt out of the spark header

[paulbalandan]

Description
Adds #[Command(headerless: true)], which suppresses the spark header for that command, with the same effect as always passing --no-header. A command whose output is read by another program, such as JSON piped to jq or CSV redirected to a file, otherwise relies on every caller passing the flag, and one caller without it breaks the output from the first line. help <command> and <command> --help still print the header.

Deciding this requires the command registry before the header is printed, and building the registry runs discovery. The duplicate legacy/modern name warning printed during discovery now goes to STDERR, so it neither precedes the header on STDOUT nor ends up in a headerless command's output.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[Copilot]

Copilot review overview

🟡 Changes recommended

The compatibility-sensitive STDERR migration lacks the required 4.8 upgrading-guide entry.

Review effort: Balanced
Findings: 1

Open (1)

• Document STDOUT-to-STDERR migration in upgrade_480.rst · New

What changed in this PR

Adds an opt-in headerless flag for modern Spark commands targeting CodeIgniter 4.8.

Changes:

• Adds headerless command metadata and lookup APIs.
• Suppresses headers for marked commands and aliases while preserving help headers.
• Routes duplicate-command warnings to STDERR and adds tests/documentation.

The STDERR behavior change still needs migration guidance in upgrade_480.rst.

File	Description
system/​CLI/​Console.php	Applies command-level header suppression.
system/​CLI/​Commands.php	Registers/query headerless metadata; moves warnings to STDERR.
system/​CLI/​AbstractCommand.php	Exposes isHeaderless().
system/​CLI/​Attributes/​Command.php	Adds the headerless attribute argument.
tests/​system/​CLI/​ConsoleTest.php	Tests suppression and help behavior.
tests/​system/​CLI/​CommandsTest.php	Tests registry metadata and shadowing.
tests/​system/​CLI/​AbstractCommandTest.php	Tests command-level accessor.
tests/​system/​CLI/​Attributes/​CommandTest.php	Tests attribute defaults and values.
tests/​_support/​Commands/​Modern/​HeaderlessCommand.php	Adds a headerless fixture.
tests/​_support/​Duplicates/​HeaderlessDuplicateModern.php	Adds a shadowing fixture.
user_guide_src/​source/​cli/​spark_commands.rst	Links headerless-command guidance.
user_guide_src/​source/​cli/​cli_modern_commands.rst	Documents the feature and API.
user_guide_src/​source/​cli/​cli_modern_commands/​017.php	Adds a JSON-output example.
user_guide_src/​source/​changelogs/​v4.8.0.rst	Records the enhancement.

Validation: no local tests were run; CI remains authoritative.

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