docs: correct stale statements, and tidy file-local exports #48

A pass over the docs against the current code, plus a small code tidy. In the docs:

• DEVELOPING.md moves to docs/, which fixes the README's link to it and its own links to DESIGN.md and AGENTS.md.
• PRs with no revision "get no verdict" (the outlined pill) instead of "stay grey".
• The CI recheck is "after about an hour" instead of "every fourth scheduled refresh", which stopped being true when the poll began running every few minutes.
• Muting is added to the README's "Using it".
• attention::check_index becomes checkIndex.
• The atgc 0.16 notes are updated for atgc 1.0, which can set the spindle (atgc repo edit --spindle).

Removed from DESIGN: the Rust-helper history, and the Publishing steps for a repo that was published days ago (now two sentences). RELEASING's "not tested yet" says why: the repo's manifest names no spindle, so CI has never run. Setting one is the first P1 in the backlog. Removed from the backlog, because they're done: publishing the repo, a push to any branch refreshing twice (#38), and the bare-patch item (folded into the matching upstream entry). The "errors can show full URLs" item is also gone: transport errors were already stripped, and this PR keeps the query string out of the last message that had it.

In the code, six names that only their own file uses are no longer exported, and two comments that described behaviour by reference to the old Rust helper are rewritten. No behaviour changes: tests (121) and every hook pass.

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