Own Whiteboard config migration and typed batch saves

[sidkmenon]

Whiteboard currently owns a TOML editor and config migration code. Add two native commands so clients can migrate saved Whiteboard settings and save a typed settings patch without editing TOML themselves.

• diffr config migrate --json converts the v1 settings exposed by Whiteboard to v2. It keeps saved toggles, context lines, hide settings, summary credentials/options, and custom prompts. Historical stock prompts follow the new default. It drops custom plugins, pipeline order, grouping, instances, and other v1 plugin fields outside that UI; theme and diff limits remain. Missing/current files are unchanged.
• diffr config set --stdin --json accepts a nested JSON partial config. Provider resets run before explicit patch values, so a provider and its credentials save together. Omitted fields remain, and rejected batches write nothing. The existing single-key command and TUI keep using the same writer.

Both commands validate the candidate in memory, serialize native writers with a file lock, and replace the file atomically. They create no backup. JSON failures include the cause, config path, and a copyable coding-agent repair prompt; TOML diagnostics omit source excerpts that could contain credentials.

Validation: full cargo test --locked passed; final config tests passed (56); CLI tests passed (13). Whiteboard's consumer branch passed 33 settings/startup tests, 12 runtime integration tests with the local candidate binary, server typecheck, lint, and format checks.

AI assistance: implemented with OpenAI Codex.