ADR-026: PR readiness and auto-merge criteria

A pull request is opened when it is complete and reviewable: no partial implementation, no TODO, FIXME or debugging left in, task quality:verify, task test:unit and task docs:validate passing, and the live tests that cover what it changes run against the local stack. CI runs the rest.

A pull request into next that changes documentation, or anything a user or a caller of bb meets, waits for a maintainer to merge it. That covers the docs pages, the README and the decision records; commands, flags, arguments and help text; text and JSON output, exit codes and error messages; MCP tools, configuration keys and environment variables; and what the packages install. A pull request that mixes the two kinds is of this kind. Any other pull request into next, to tests, CI, the local stack or internal code with no visible effect, may merge itself by rebase auto-merge, which lands it once CI Complete passes. Resolve its review comments before turning auto-merge on.

A maintainer reviews what users see. Everything else lands on next, which releases nothing, and is looked at as a whole when next is promoted. Rebase auto-merge keeps the history linear and leaves the waiting to the required check.