Sleekplan MCP: allow create / update / delete of custom meta on existing posts

Description: Add MCP tools to create, update, and delete custom meta on existing feedback posts — not just at creation time. Today the Sleekplan MCP can only set meta when a post is created (createfeedback exposes metaorigin and meta_service). There is no way to: add or change meta on a post that already exists, or set arbitrary meta keys beyond origin / service (e.g. a reporter key), or remove a meta key/value. Problem: Posts created earlier (or created by other integrations) can't be enriched with meta afterwards — e.g. backfilling a reporter email, source channel, or account ID. Only two meta keys (origin, service) are writable, so anything else has to be crammed into those or into the description. listfeedback already supports filtering by custom meta / metasystem, so the read side expects meta that the write side can't fully produce — an asymmetry. Suggested solution: Extend updatefeedback with a meta parameter (arbitrary key/value pairs), or add dedicated tools such as setfeedbackmeta / deletefeedback_meta. Support arbitrary meta keys (not just origin/service) on both create and update. Allow deleting a meta key (e.g. pass null/empty to unset). Benefit: Enables enriching and correcting post metadata over time (reporter, source, account, campaign, etc.), makes the write side consistent with the meta-based filtering already available in list_feedback, and unblocks agent/automation workflows that classify or attribute posts after creation.