Bump 0.21.1 -- post-process exit-code propagation

[moodmosaic]

Summary

• Cut 0.21.1 to release and tag the post-process exit-code propagation that merged in Propagate post-process container exit code #105 — it shipped to master without a version bump.
• Document the new contract so callers (CI workflows, daemons) know post-process/wait now gate on the container's exit code.
• Docs + version only; no code change in this PR.

Changes

• CHANGELOG.md: add a ## 0.21.1 section describing the exit-code propagation fix (harvest still runs; non-zero container exit now propagates).
• USAGE.md: note in the post-processing section that ./launch.sh post-process (and wait) exit with the container's code after harvesting.
• VERSION: 0.21.0 -> 0.21.1.

Test plan

• ./tests/test.sh --unit — 1357 passed, 0 failed.
• Version gate (mirrors CI): CHANGELOG.md contains ## 0.21.1, and 0.21.0 -> 0.21.1 increments under sort -V.
• After merge: git tag -a v0.21.1 <merge> -m "v0.21.1 -- post-process exit-code propagation", then git push origin --tags.