Bumps the package version to 0.1.2 and writes the CHANGELOG section
for the docs/hexdocs overhaul (PR #20): ADRs unpublished from hexdocs,
ex_doc pinned to ~> 0.40 with zero mix docs warnings, README badge row
and documentation index line. No library code changes.

Also corrects .claude/wurk/commit.md, which still described the
version field as 0.1.0-dev, to name the last released version.

Tagging and hex publish stay with the operator.

Refs: sp-sv5