task: decide on MPL-2.0 source-file header convention

[martinydeAI]

Surfaced from ADR 002 (project license: MPL-2.0)'s original
"Follow-up Actions" list as part of the cleanup tracked in #44.

ADR 002 noted, as an optional follow-up, that new source files going
forward could carry the short MPL-2.0 Exhibit A notice at the top:

// This Source Code Form is subject to the terms of the Mozilla
// Public License, v. 2.0. If a copy of the MPL was not distributed
// with this file, You can obtain one at https://mozilla.org/MPL/2.0/.

Existing files inherit the project-wide LICENSE, so this is purely
about a per-file convention going forward.

Decide one of:

1. Yes, require it on new source files. Document the convention
   in CLAUDE.md and optionally add a php-cs-fixer rule or
   pre-commit check to enforce it.
2. No, rely on the repo-level LICENSE only. Document the
   non-decision in CLAUDE.md so future contributors don't add the
   header inconsistently.

Refs ADR 002 (docs/adr/002-project-license-mpl-2.md).

[turegjorup]

From a personal standpoint I don't care. My AI buddy produced this summary on the issue:

Quick Answer

Whether you are legally required to include a copyright notice header in every file of an
open-source repository depends on the specific license and your project's goals. There is
no universal legal requirement that mandates a header in every single file for copyright
protection to exist.¹
However, here is a breakdown of the best practices and considerations:

Legal Necessity vs. Best Practice

While copyright protection is generally automatic upon creation, including a notice in each
file is considered a best practice.⁴ It ensures that if a file is moved, copied,
or used in isolation, the licensing terms and ownership remain clear to anyone who
encounters it.⁵

Project-Specific Requirements

• GNU Projects: The Free Software Foundation (FSF) explicitly recommends that you include
  a copyright notice and a license statement in every "nontrivial" file (defined as any file
  longer than ten lines).⁷
• Apache Projects: The Apache Software Foundation has specific policies regarding source
  headers to ensure that licensing information is properly propagated.³

Modern Alternatives

Many developers now use SPDX Identifiers (e.g., SPDX-License-Identifier: MIT) as a
concise way to indicate the license of a file without needing a long, verbose
header.¹

Maintenance Considerations

• Managing copyright years in every file can be burdensome as a project grows.⁴
• Some developers choose to use automation tools to manage these headers across a repository
  to ensure consistency without manual overhead.⁵

Summary Recommendation

While not strictly required by law for copyright to exist, including at least a license
identifier or a brief header in each file is highly recommended to protect your
project's legal clarity and to ensure that downstream users understand the terms under
which the code is distributed.

References

The *SPDX Identifiers** (e.g., SPDX-License-Identifier: MIT) options sounds like a good sweet spot for me