Commons: finish poll and vote as a group's non-binding voice #425

Problem

Core encodes what a group stack's owner and admins can do, and nothing about members' say in the group's direction. Enforcing approval rules in core wouldn't help yet: whoever acts as the owner bypasses any rule core enforces, until #422 settles who that is.

What members can have now is voice as data. org.haverstack/poll@1 and org.haverstack/vote@1 already exist in docs/commons/poll.md, marked proposed and awaiting an intended writer.

Decision

• Proposals and votes are ordinary commons Records. They bind nothing in core; each group decides what a vote means.
• Finish the pair through the commons governance process, with a concrete intended writer: a group-decisions app, or an example app in this repo.

Actions

• Identify the intended writer.
• Review poll.md against the commons design rules, and settle anything still open in it.
• Move both types from proposed to defined, and add them to @haverstack/commons.
• Changeset for @haverstack/commons (minor: new exported types).