Give reviewers base-pinned AGENTS.md and CLAUDE.md for the changed paths #682

Problem

Repository guidance files (AGENTS.md, CLAUDE.md, nested AGENTS.md files, and the docs/ they link) are where many repositories state the rules a change must follow. Today they reach a reviewer only when an agent prompt tells it to open them in the checkout:

• cr does not read them; the dossier's repo-guidance.md covers only .codereview/agents provenance (internal/dossier/dossier.go, "Separate review-guidance files are not used for this review").
• With Keep claude_cli and codex_cli reviewer sessions independent of ambient instruction files #681, no adapter loads them implicitly either, which is the right default for isolation.
• The checkout a reviewer reads is the PR head. When a PR edits a guidance file, a reviewer that follows its prompt reads the edited rule, so a PR can loosen the rule it is judged against. .codereview/agents already avoids this by reading from the base branch.
• Each prompt has to rediscover which guidance files apply to which changed paths, and prompts differ in how well they do it.

In repositories that pair their AGENTS.md rules with a repo-local reviewer naming those rules, a meaningful share of findings cite the guidance document and section directly, so this input carries real review signal when it is present.

Proposal

Add a reviewer-facing dossier artifact, for example dossier/final/guidance-map.md, built from the pinned base SHA:

• For each changed path, list the guidance files that apply: root AGENTS.md and CLAUDE.md, and every AGENTS.md or CLAUDE.md in the path's ancestor directories.
• Store the base-branch content of each listed file under the run artifact directory, so reviewers read the trusted version rather than the PR head's.
• Mark guidance files the PR modifies, with a pointer to the diff, so a reviewer can judge the edit itself.
• Keep it a map plus file copies, not prompt text: reviewers read what they need, in line with the checkout-native design that avoids context stuffing.

Out of scope: following links from guidance files into docs/; agent prompts can still do that.

Open question

This changes the context every reviewer sees by default. It should be measured on a fixed PR set before it becomes the default, comparing findings that cite repository rules and false positives with and without the map. An opt-in profile or flag for the trial would make that comparison straightforward.

Acceptance

• Dossier artifact listing applicable guidance files per changed path, with base-SHA content copies and modified-by-PR markers.
• Guidance files changed by the PR do not alter the copies the reviewers receive.
• Measured comparison recorded before enabling by default.