Single-source the version number

[AngriestBird]

Bottom line

Six places carry a version and they disagree. The Windows binary reports 2.0.0.0 whatever
tag it was built from.

Why

• pyproject.toml:7 version = "0.1.0"
• src/hoi4cm/__init__.py:5 __version__ = "0.1.0"
• build/version_info.txt:8-9 filevers=(2, 0, 0, 0), prodvers=(2, 0, 0, 0), and
  :25/:30 u'2.0.0.0'
• README.md:8 ![Version](...Version-2.0-gold)
• hoi4_content_maker.py:4 # Version 2.0 | Millennium Dawn Team
• CHANGELOG.md:423 ## [2.0] — 2025, the newest versioned heading

Nothing derives any of these from anything else. AGENTS.md:54 says the release version
is the tag name, and the release job uses github.ref_name for the Release title only
(ci.yml:149); build/build.py never rewrites version_info.txt. So a v2.1.0 release
ships an executable whose Properties dialog reads File version 2.0.0.0, while
pip install . reports 0.1.0.

Acceptance

• One source of truth for the version; the rest are generated from it or removed.
• The built executable's embedded version matches the tag it was released from.

BLUF