refactor: classify release plan failures

[vitormattos]

Summary

Refactor release:plan so expected planning failures are handled explicitly instead of catching every DomainException.

The successful ReleasePlan JSON contract and release planning rules stay unchanged.

Unexpected Git or infrastructure failures are no longer reported as invalid user input. They remain visible as unexpected failures.

Regression coverage

• keep request validation behavior
• keep JSON error format for invalid configuration and planning rules
• keep exit code 3 for valid plans that are not ready
• keep decorated milestone matching
• keep backport blockers and explicit override behavior
• keep proposed patch version behavior
• verify unexpected DomainException is not masked
• exercise the CLI contract with the real planner and in-memory Git/GitHub boundaries

Release context

This is being stabilized before the next release-tool version is consumed by LibreSign stable33, stable34 and stable35.