docs/README.md index misses a dozen published docs, including BACKENDS.md

\`docs/README.md\` lists 11 documents. These published docs are not reachable from it: \`\`\` docs/BACKENDS.md docs/desktop.md docs/DISTRIBUTION.md docs/COST_LEVERS.md docs/NIGHTLY_EVAL.md docs/NORTH_STAR_BENCH.md docs/REVIEWER_RECALL_METHOD.md docs/UNTRUSTED_PR_REVIEW.md docs/ARCH_REVIEW.md docs/UI_AUDIT.md docs/BASELINE_M0.md docs/design/memory-lifecycle-triage.md \`\`\` \`BACKENDS.md\` is the visible gap: \`docs/configuration.md\` sends readers there for \`worker.backend\`, but neither the index, the quickstart nor the README link it. **Fix:** add rows for at least \`BACKENDS.md\`, \`desktop.md\`, \`DISTRIBUTION.md\` and \`COST_LEVERS.md\` to the table (a second "Reference / reports" table for the audit-style docs is fine). Docs only, ~12 lines.