feat: enforce per-file size limit in files.write tool handler (#12)

[mgoldsborough]

Summary

• The API-layer `bodyLimit` middleware caps HTTP uploads, and `src/files/ingest.ts` enforces `maxFileSize` on the user-upload path. The `files__write` tool handler reached by the agentic loop does not traverse either — so the "no file larger than `maxFileSize`" invariant did not hold at the point of write.
• Plumbed `runtime.getFilesConfig().maxFileSize` into `handleWrite` and reject the call with a descriptive error when the decoded payload exceeds it.
• Reads from runtime config (not a hard-coded constant) so operators who raise the limit in `nimblebrain.json` see it applied consistently across the HTTP and tool-call paths.

Test plan

• New `test/unit/tools/platform-files.test.ts` — 3 tests covering accept-within-limit, reject-over-limit (error message includes filename and mentions the limit), and respects runtime config override
• `bun run lint` / `bun run check` — clean

Closes #12