docs(kits): note the accepted extension-to-kit divergences

[CorieW]

Writes the Notes entries for the accepted divergences in #3035, the ones with no kit equivalent to restore. Four are platform-level, so they go in kits/README.md under "Migrating from an extension" rather than being repeated in every kit; the per-kit specifics go in the kit that has them.

Changes

• kits/README.md: new "Differences that apply to every kit" section covering the ext- to kit- resource-name prefix, the missing billingRequired declaration, the missing deploy-time status surface, and validation moving from the install prompt to deploy time.
• firestore-bigquery-export: note that initBigQuerySync / setupBigQuerySync only report to logs, where the extension's install and update hooks set the instance's processing state.
• firestore-incremental-capture: note the same loss for runInitialSetup and onFirestoreBackupInit.
• firestore-translate-text: note that the extension's backfill status counts went with the backfill.
• firestore-genai-chatbot: note that validateRequiredEnvVars() is gone, that only PROJECT_ID still has anything to check, and that the rest is enforced by the CLI at deploy.

No entry added for NODE_PATH to RTDB_NODE_PATH or the ext- to kit- topic rename: rtdb-limit-child-nodes, delete-user-data and bigquery-firestore-export already document those. firestore-vector-search already notes its missing backfill progress reporting.

Two corrections to the ledger's list while writing these. storage-resize-images is not affected by the status surface: its four setProcessingState calls are commented out (functions/src/index.ts:213-278). firestore-incremental-capture is affected and was not listed (5 calls in functions/src/tasks/). billingRequired: true is declared by 11 of the 13 predecessor extensions, not just firestore-translate-text and rtdb-limit-child-nodes; speech-to-text and firestore-bundle-builder never declared it.

Docs only, no code change. prettier --list-different is clean.

[gemini-code-assist]

Code Review

This pull request updates the documentation across several kit READMEs to detail platform-level differences between Firebase Extensions and the new kits. Key updates explain the new kit- resource prefix, the absence of billing declarations, the shift of status reporting to function logs, and the transition of configuration validation to deploy time. The feedback suggests a minor phrasing improvement in the main README to enhance readability regarding resource reuse.