fix(pr-review): a pull request session reads its own repository - #2478

🟢 Confidence 8/10 · likely safe to merge. Pinning and withholding are pinned by tests; the untested piece is the new unreadable fail-fast in turn-runner, whose reply-row write drops the reason.
_{1 note · tests partial −1 · risk high −1 · 1/1 new units observable}

A review or reply session now pins every repository-naming tool to its own pull request's repository, withholds those tools when it cannot be read, and fails an unattended pass instead of running unpinned. The scope change holds; one copy defect on the reply path.

• buildChatToolkit overwrites repository for the six repository tools and withholds them when the scope is unreadable
• turn-runner resolves replyTarget and fails an unattended pass whose pull request cannot be read
• list_source_repositories leaves the reviewer's ruleset and joins the code-mode tool set
• search_source_code rejects a path containing whitespace, closing the qualifier-injection vector

Findings

Give `UnfinishedRows.failReply` the message argument that `failInvestigation` has, and pass `PULL_REQUEST_UNREADABLE` as the reply row's `error` and as its posted notice.

Production impact

_{8896f53 · Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}