prototype: explore read-only release planning

[vitormattos]

Important

Prototype only — do not merge as the production implementation.

This PR was created to explore the read-only planning contract. The production architecture is now defined by #70 and #78 and is implemented as the PHP CLI/PHAR in LibreCodeCoop/release-tool, with github-workflows providing thin orchestration. #71 remains open until that production implementation satisfies its current contract.

Prototype scope

This PR explored:

• a read-only release planner;
• composite-action wrapping;
• branch/version discovery;
• immutable planning output;
• unit-testable Python logic.

Its behavior may be reused or rewritten, but this branch/PR is not the source of truth.

Superseding contracts

Follow:

• Define the reusable release engine architecture and stage contracts #78 for architecture/runtime/module boundaries;
• Support alpha, beta, RC and final release channels #80 for prerelease/version state;
• Support per-major Keep a Changelog storage as a reusable changelog strategy #79 for changelog strategy;
• Plan and validate a selected release with the reusable engine #71 for the current planner contract.

The old multi-stable assumptions and simplified patch-only behavior in this prototype must not be carried forward unless they match those issues.

Disposition

Keep this PR only as disposable reference material while useful behavior/tests are extracted into the PHP production implementation. Do not merge it. Close/delete it once the relevant production contracts have equivalent or better coverage.