Support a private flag on comments (internal / admin-only notes)

Description: Add a private flag to createcomment (and to updatecomment, so a comment can be flipped after the fact) that marks the comment as internal — visible to admins only, not to end users on the board. Today createcomment accepts only feedbackid, comment, parent and pinned. There is no visibility argument, so every comment written through MCP is public. pinned is the only visibility-adjacent control and it points the other way: it promotes a comment to the top of the thread. Problem: Agents that triage feedback have nowhere to put internal context — the Intercom conversation a request came from, a duplicate assessment, a "we tried this in Q2, here's why it didn't ship" note, an effort estimate. The options are to write it publicly on the customer-facing board or to not write it at all, and both are bad. Internal reasoning ends up outside Sleekplan, which is exactly the split the MCP server is meant to close. Benefit: Agents can leave triage notes on the post where the context belongs instead of in a side channel. Public threads stay clean; customers see the answer, not the deliberation behind it. The internal record travels with the post through merges and status changes. Also worth deciding — the read side: It is currently unverified whether listcomments returns private comments at all, and if so whether the payload carries a visibility flag. This needs to be explicit either way: an agent that silently reads internal notes and repeats them back to a customer is a worse failure than not being able to write them. Whatever the write flag ends up being, listcomments should return the same field so callers can tell the two apart. Related: the author request on create_feedback (#299114) — same underlying theme of MCP-created content being second-class compared to what the portal can produce.