docs: add changelog and v1-to-v2 migration guide

[antoleod]

Summary

Closes #275

• add a Keep a Changelog-style CHANGELOG.md with an Unreleased section and release-link convention;
• add docs/migration/v1-to-v2.md with the four-step v2 migration process described by the README;
• turn the README versioning references into links to the new migration guide and changelog.

Scope

Documentation only. No runtime code, API behavior, dependencies, configuration, source files, or tests changed.

Verification

• acceptance-criteria preflight passes;
• changed-file scope is restricted to README.md, CHANGELOG.md, and docs/migration/v1-to-v2.md;
• npx prettier --check README.md CHANGELOG.md docs/migration/v1-to-v2.md;
• git diff --check;
• lint/build/test are compared against a clean upstream/main baseline so this docs-only patch cannot turn a previously passing check into a failure.

At submission time, upstream main already has unrelated failures in lint/build/tests. Those checks are therefore baseline-compared rather than represented as green. This patch adds no source or test changes.

Acceptance criteria

• CHANGELOG.md includes Unreleased and a release-linking convention.
• docs/migration/v1-to-v2.md mirrors the four migration steps from the versioning policy.
• README links resolve to the new files.

Bounty

Submission for issue #275 — $100 bounty.