Release 2.0.0 #430

Cut v2.0.0 from `release/2.0` as it stands: v2.0.0-rc.8 plus the README split (#416).

• `pyproject.toml` → `2.0.0` (and the `cldk` entry in `uv.lock`, which still reads `2.0.0rc7`)
• `CHANGELOG.md`: a `v2.0.0` section summarising the 1.x → 2.0 breaking changes, each with its migration step, and pointing back to the rc sections
• PR into `release/2.0`, merge, then tag `v2.0.0` on the merge commit

Out of scope, for 2.0.x: #421 (Java duplicate-FQN fix) and the analyzer pin bumps (#413, #423, #427, #428, #429).