feat: add PR reviewability analysis and scope monitoring

[kieranklaassen]

Summary

• New agent: pr-reviewability-analyst — scores PRs on reviewability (0-100) using weighted line counts, file counts, and concern analysis. Proposes concrete stacked PR splits when score < 60.
• ce:review integration: Added Step 1.5 "Pre-Flight Reviewability Check" that runs before all other review agents. Non-blocking (P2 finding). Updated summary report template.
• ce:work integration: Added "Scope Check" to the task execution loop. Measures total branch divergence after each task. Soft warning at 300 lines/5 files, hard warning at 500 lines/10 files with checkpoint + stacked branch flow.
• ce:work-beta parity: Mirrored the same task-completion and scope-check behavior into ce:work-beta so this guidance does not get lost if ce:work-beta is later promoted to stable.

Motivated by EveryInc/proof#536 — a 172-file, ~50K-line PR that was closed because it mixed too many concerns and introduced 3 independent regressions across unrelated areas.

Key Design Decisions

• Non-blocking: Reviewability check is P2 (informational), not P1. Teams adopt gradually.
• No automated splitting in v1: Agent outputs a structured split proposal. Automated cherry-picking is a v2 enhancement.
• Hardcoded pre-flight: Always runs during /ce:review, not part of user-configured agent list.
• Branch divergence measurement in ce:work/ce:work-beta: Total diff from default branch, so incremental commits don't reset the counter.
• Stable/beta workflow parity: ce:work-beta keeps the same reviewability guardrails as ce:work, while preserving its beta-only delegation guidance.
• Warning fatigue prevention: After user declines a hard warning, suppresses until 100+ additional lines.

Testing

• Verified all JSON files valid (jq .)
• Verified ce:work-beta mirrors the new ce:work scope-check/task-completion guidance while preserving beta-only delegation sections

Post-Deploy Monitoring & Validation

• No additional operational monitoring required: plugin content change only — no runtime code, no deployed services

Follow-up

• Update setup skill to include reviewability defaults in new settings files
• Run /release-docs to rebuild documentation site

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[tmchow]

@kieranklaassen rebased this branch onto origin/main to resolve the merge conflicts with main given a bunch of merges I've done. Didn't want to saddle you with rebasing and such.

I also mirrored the new ce:work task-completion and scope-check changes into ce:work-beta so that if ce:work-beta is later promoted to stable, this reviewability work does not get lost.