rtdb-limit-child-nodes: README documents RTDB_NODE_PATH and MAX_COUNT defaults that no longer exist

The kit README says the kit "falls back to its default of `messages`" when `NODE_PATH` is copied from an extension config (README.md:126-131), and has a section "`MAX_COUNT` now defaults to 100, and 0 is rejected" claiming both params have defaults so an incomplete config deploys (README.md:136-140).

Neither default exists. c4735460 (2026-08-24) removed them for parity: the extension declared both params required with no default, and a deploy with no `MAX_COUNT` had pruned every watched node down to 100. The README text is from 4c011e29 (2026-08-17) and was not updated.

Current behaviour on `kits` at 9cc453c5: `kits/rtdb-limit-child-nodes/src/config.ts:47-78` declares both params without a default, so the CLI prompts for them; `src/export-config.ts:46-51` throws at runtime on an empty path or a non-positive count.

Fix is docs only: drop the `messages` fallback sentence, and replace the defaults section with the one thing that is still true, that `MAX_COUNT` of 0 passes the extension's regex but is rejected at runtime.

Read README.md:126-140 and compare its claims with kits/rtdb-limit-child-nodes/src/config.ts:47-78 and src/export-config.ts:46-51. Remove the nonexistent defaults and document only the runtime rejection of MAX_COUNT=0; done when the README matches the current configuration and validation behavior.

Written by the indexing model from the issue text.