docs: move CLAUDE.md to AGENTS.md, and reconcile the backlog

AGENTS.md is the single source of truth for how to work on this
repository, shared across every agent tool; CLAUDE.md is now a thin
pointer to it, mirroring the arrangement in the user's own config.
Prose throughout the specs, plans, source comments and tests refers to
this document as CLAUDE.md, which is what it was called until now.
Those references are historical records and are correct about the
content, so they are left alone and AGENTS.md says how to read them.

Two rules added while reconciling:

- The reconcile writes its open-items table to the user's vault as well,
  rewritten whole each time so a closed item cannot linger there.
- Build a test only when it asserts something measurable, and hand a
  visual change to the user instead. This is the conclusion the
  "Rendering probes lie" section was already building toward, and it
  sits at the end of it. Guarded against the opposite reading: it does
  not license dropping a test that can fail, and items 87, 105 and 176
  are each a case where the missing test was the whole problem.

The 2026-08-29 pass over the user's notes found two entries with no item
here, both causes verified in the code rather than copied from the note:

- 179, undo is one level deep in practice and there is no Redo. The
  QUndoStack is real and multi-level; what is missing is a redo action
  and an answer to the stack being cleared on every new query.
- 180, the repaint rules are discovered one hole at a time. A question
  rather than a defect, and the user's own framing: items 105, 107, 109,
  110 and 170 are each one hole in the same surface, all found by hand.

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