docs: correct the flake-attempts semantics and fill the review gaps

--flake-attempts is total attempts, not retries: ginkgo v2.29.0 sets
maxAttempts = FlakeAttempts and loops attempt < maxAttempts, and the flag's
usage string reads "0 - failed tests are not retried". At 1 there is no retry
at all, so "retries a failing spec once" was false in CONTRIBUTING.md and
implied in .agents/building-and-testing.md. Both now say each spec runs once,
and cite the source so the next reader need not re-derive it.

Also restores the React-UI stub rationale, which is load-bearing because a spec
asserting on a UI asset passes locally against a real dist/ and is served the
stub in CI; explains why 213 and ~240 differ; records that the workflow also
triggers on master pushes, where paths-ignore does not apply; and completes the
LOCALAI_E2E_REQUIRE_BINARIES value table, including that any unparseable value
reads as ON.

In .agents/ci-caching.md the stale "13 of those 20" figure now carries its
qualifier inline rather than in the following sentence.

Assisted-by: Claude Opus 5 [claude-code]
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>
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