Version Your Harness and Track Behavior

PROJECT CONTEXT:
Someone edits the system prompt, the agent gets weirder, nobody knows why or when. Version-evolution research on the leading harness shows behavior tracks harness changes tightly - which means harness changes deserve versioning, changelogs, and behavioral diffs.

GOAL:
Put the harness under behavioral version control: versioned prompt/tool/config artifacts, a behavior changelog, and a smoke eval that diffs behavior across versions.

REQUIREMENTS:
- All behavior-bearing artifacts (prompts, tool descriptions, rules, routing) in version control as reviewable files
- A harness version identifier stamped into logs and session records
- A behavior changelog: every change entry says what behavior is expected to differ
- A fixed smoke-eval suite run before and after changes, with diffs reviewed
- Rollback: any prior harness version reproducible on demand

CONSTRAINTS:
- No behavior-bearing string lives outside versioned files - including 'temporary' prompt tweaks
- Changes without a changelog entry do not merge

DEFINITION OF DONE:
- Prompts and tool descriptions live in files with review history
- Session logs carry the harness version that produced them
- The changelog explains the last five behavior changes in behavioral terms
- A prompt change shows its behavioral diff via the smoke eval before merging

COMMON FAILURES TO AVOID:
- Prompts as unversioned strings buried in code, edited live
- Changelogs listing file edits instead of behavior changes
- No baseline eval, so behavioral drift is discovered by angry users
- Sessions unattributable to harness versions when debugging regressions