fix(reviewer): split agy prompt into trusted AGENTS.md, real argv, and a diff file

[asavs]

Supersedes #174

#174 fixed the MAX_ARG_STRLEN argv crash (staging the entire untrusted payload behind a generic --print pointer) but traded it for two new problems, both found during review of that fix:

1. It buried the angry personality's narrative "prefill" inside a file read via a tool call instead of the model's actual literal prompt — weakening a device that only ever worked because the payoff line was the last thing the model read before generating.
2. append_ci_coverage_context was still inlining ~39–96KB of raw workflow YAML / package.json content into every review regardless of PR size — itself a quarter of what made PR M0: Add launch validation before enabling cron/live daemon #38's prompt oversized in the first place.

This PR redesigns the split three ways instead of patching #174 further:

• AGENTS.md holds only genuinely trusted, daemon-authored content. The angry personality's simulated dialogue (append_angry_agents_prefill) is removed entirely — a Rules file is static instructions, not a conversation. Confirmed via agy's own bundled docs that AGENTS.md is a real, documented Rules-loading feature (auto-discovered by exact filename, walking up from cwd), not a workaround.
• The literal --print argv value is a real, PR-specific prompt again. PR metadata, commit subjects, prior review/threads, and the angry interruption/prefix/tail are reunified into one continuous string assembled in agy.sh — small and bounded by construction (new REVIEWER_MAX_ARGV_PROMPT_BYTES, default 100000, plus a hard ~130000-byte backstop against the real kernel limit right before exec). Only the diff — the one section that can legitimately run large, and the one thing the model can't self-serve since the PR-head snapshot has no .git history — goes to a separately staged file (task_dir/REVIEW_DIFF.md).
• CI coverage context is no longer inlined at all. append_ci_status (GitHub API pass/fail facts) stays; the raw YAML/JSON dump is replaced by one sentence in append_source_snapshot_hint pointing the model at .github/workflows/ and package.json to read itself if a finding needs it — the same explore-yourself idiom already proven for the PR-head snapshot generally.

Verification

• Full fixture suite: 633/633 (up from 617) under WSL, the project's real GNU/Linux test runtime.
• Live-VM dry run against asavs/mog-template#38 — the diff-heavy PR that's been silently failing to get reviewed since it opened: AGENTS.md dropped from what would have been ~45KB to 6.5KB, the reunified --print argv came in at 1268 bytes for a 112KB diff, and the model produced its first real review verdict on that PR (APPROVE), citing specific files by path from the separately-staged diff.

Test plan

• bash -n on every touched shell file
• bash scripts/reviewer/tests/run-fixtures.sh — 633/633
• git diff --check
• Throwaway sanity check: every argv-bounded section pushed to its configured maximum simultaneously stays at ~11.9KB, far under both the 100000-byte argv budget and the 130000-byte hard kernel-limit check
• Live dry run against a real diff-heavy PR, inspecting the dry-run artifact's AGENTS.md/prompt/diff sections directly