docs: record 0.1.0 in the changelog on main

[RobertoIskandarani]

Summary

Post-release bookkeeping for 0.1.0, and the two runbook steps that turned out to be missing while shipping it.

Changes

• CHANGELOG.md: the ## [0.1.0] heading only ever existed on the tag, because release.yml deletes the release branch after tagging. This records it on the default branch and opens a fresh empty ## [Unreleased] above it. The ## [0.1.0] section is a byte-for-byte copy of the tagged one — verified by diffing both against each other.
• RELEASE_SETUP.md §5: added steps 5 and 6. Step 5 is the changelog record above, with the rule that corrections after the fact go in the GitHub Release body rather than in the recorded section. Step 6 is the manual merge of the next-dev bump PR, which does not auto-merge because allow_auto_merge is off on this repository. Step 4 also now mentions that the nuget environment requires a maintainers approval before anything reaches nuget.org.

Test Plan

Documentation only; no code, workflows or packaging touched.

• The recorded ## [0.1.0] section was diffed against git show v0.1.0:CHANGELOG.md — identical, 212 lines.
• ## [Unreleased] is present again on the default branch, which cut-release.yml requires for the next cut.

Checklist

• dotnet build / dotnet format / dotnet test — n/a, no source or project files touched
• Tests added for new functionality — n/a
• Documentation updated
• CHANGELOG.md entry added — this PR is the changelog entry
• New workflow actions are SHA-pinned — no workflow changes
• No token values, secrets, or key material in logs or test fixtures