Weekly client report in Slack

Read https://www.sellermate.ai/api-docs first. It documents the SellerMate Public API: REST over HTTPS, X-API-Key auth, and every tool with its parameters is on that page. Every tool is POST https://api.sellermate.ai/public/v1/tools/{name} with a JSON body.

Build me a weekly client report that runs every Monday at 8am and posts one Slack message per Amazon account.

Each message should show, for the last 7 days versus the 7 before:
- Spend, sales, ACoS and orders, with the percentage change
- The three campaigns whose spend or ACoS moved most, with the numbers
- A one-line plain-English summary a client can read in ten seconds

Use get_user_context to list accounts, get_marketplace_summary for the account totals, and get_campaign_performance with sort_field set to cost for the movers. Post to the Slack webhook in SLACK_WEBHOOK_URL. Give me a --dry-run flag that prints the messages instead of posting.

[Optional: only include accounts whose name contains "Client".]

Rules:
- My key is in the SELLERMATE_API_KEY environment variable. Send it as the X-API-Key header. Never hard-code it or log it.
- Call get_user_context first and use the accounts it returns. Don't invent account ids.
- Report tools are paginated (page, page_size up to 500, pagination.hasMore). Walk every page.
- On a 429, wait for the Retry-After header and retry. On a 401 or 403, stop and tell me.
- Show me the plan and the code before running anything that writes.

Stack: Python 3 with requests, no framework. Include a README with the cron line.