workflow-fix: code-reviewer diff-size budget on long-lived branches

Overview / Motivation

Auto-filed by the workflow-fix-on-bug protocol from a workflow-fix candidate raised on task #722 (emitting agent: orchestrator, /issue 722 session).

Goal

code-reviewer Step 0 sizes the branch diff before reading it and falls back to per-round-commit git show when it exceeds a byte budget.

Workflow gap

• Bug observed: two subagent spawns on #722 r2 (an experiment-implementer, then a code-reviewer) died on autocompact thrash after loading the whole-branch diff — on that 4-day-old same-issue-follow-up branch git diff main...HEAD is 1.96 MB (43K insertions, 136 files of prior ALREADY-REVIEWED rounds) and main..HEAD is 31.6 MB. The orchestrator had to measure the bomb and re-spawn the reviewer with a hand-written command whitelist.
• Why it is a workflow gap: .claude/agents/code-reviewer.md (Step 0, line ~72 git diff --name-only main...HEAD + the "Read every line of the diff" directive) prescribes branch-wide diff reads with no size guard; long-lived follow-up branches make the cumulative branch diff unboundedly large even when the ROUND's own reviewable delta (the fix commits) is tiny (36 KB + 29 KB on #722 r2).
• Confidence (emitter): high

Proposed change (candidate diff sketch — refine in planning)

In code-reviewer.md Step 0 (and the round-scope guidance for revision rounds):
+ Before reading any diff body, size it: `git diff main...HEAD | wc -c` (name-only/stat forms stay unrestricted).
+ If the branch-wide diff exceeds ~300 KB, DO NOT read it. Scope the review to the
+ round's own commits (`git show <sha>` per implementer-marker-named commit, or the
+ `<parent>..HEAD` round range) — on revision rounds the earlier rounds were already
+ reviewed, so the branch-wide diff is redundant context by construction.
+ Never run the two-dot `main..HEAD` body on a worktree branch (includes everything
+ main advanced; 31.6 MB on #722).

Scope / surfaces

• Primary target: .claude/agents/code-reviewer.md
• Grep the workflow surface for the pattern before editing (grep -rln 'main\.\.\.HEAD' .claude/ CLAUDE.md scripts/) and update every hit that instructs reading a branch-wide diff BODY without a size check; list them in the plan. (Sibling context-hygiene gap: .claude/agents/experiment-implementer.md — the #722 implementer spawn also thrashed; consider a shared "size before you read" bullet.)

Constraints / invariants

• Workflow-surface only — never experiment code, configs/, or tasks/.
• scripts/workflow_lint.py --check-asks passes; ruff on touched files passes; if workflow.yaml or CLAUDE.md change, they stay consistent with the rule file.
• This session runs under EPM_WORKFLOW_FIX_SESSION=1 and carries a workflow_fix_target: Provenance line — it MUST NOT auto-route any of its own subagents' workflow-fix candidates (recursion guard).

Provenance

• workflow_fix_target: .claude/agents/code-reviewer.md
• fingerprint: 2a27315ebf2d

target_file: .claude/agents/code-reviewer.md bug_observed: two reviewer/implementer spawns on 722 r2 died on autocompact after loading the whole-branch diff (main...HEAD 1.96 MB on a long-lived followup branch) why_workflow_gap: code-reviewer.md Step 0 prescribes branch-wide diff reads with no size guard; follow-up branches make the cumulative diff unbounded while the round's own delta stays tiny proposed_change: code-reviewer Step 0 sizes the branch diff before reading it and falls back to per-round-commit git show when it exceeds a byte budget diff_sketch: |

• size the diff first (wc -c); >~300 KB -> review the round's own commits via git show
• never read the two-dot main..HEAD body on a worktree branch confidence: high related_task: #722