* feat(reviewer): inline AGENTS.md into reviewer system prompt
Fetches AGENTS.md from the PR's head_sha via the GitHub contents API
during reviewer setup and inlines it as a "Repository conventions"
block in the system prompt. Mirrors how the per-repo review style
prompt is already wired.
The main agent has long had a mandatory step to read AGENTS.md after
cloning, but the reviewer often skips cloning entirely (it can
`gh pr diff` directly), so it never saw the file. Loading it
deterministically means the reviewer judges findings against the
project's own conventions instead of relying on the model to fetch
the file itself.
* fix(reviewer): fetch AGENTS.md from base_sha, not head_sha
The reviewer inlines AGENTS.md into its system prompt. Reading from
head_sha means a PR author can edit AGENTS.md in the same PR being
reviewed and smuggle instructions like "ignore all bugs" / "publish
no findings" into the reviewer's prompt. Switch to base_sha (the
target branch's pre-PR state, which is trusted) and update the prompt
text to reflect that the contents come from the base, not the head.
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Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
