Spend

Pin assigned skills to a version, or unpin them (``version_id: null``). A pinned bot keeps getting that version's files, name and summary when the skill is edited. An unpinned bot follows the latest version, as before. The batch is all-or-nothing: * 404 if a skill is not assigned to this bot, or a version is not a version of that skill; * 422 if a platform skill's version is neither published nor the latest; * 409 if a version disappeared while pinning. Same permission as assigning skills to the bot. Non-blocking, like assignment: it writes the pins, then notifies **this bot only**. A device gets a manifest nudge; an OpenClaw bot gets the provisioner triggers. No other bot holding these skills is touched. Returns all of the bot's assignments.