Split the base manifest schema reader by responsibility

[codeforester]

Problem: base_setup.manifest now owns schema dataclasses, YAML loading, top-level parsing, release parsing, build parsing, health parsing, command parsing, IDE bridges, GitHub config, and artifacts in one large module. Scope: split a focused subset into sibling modules while preserving the public BaseManifest/read_manifest API. Acceptance criteria: existing imports keep working, new parser modules have focused tests through the existing manifest suite, and focused base_setup manifest tests pass.