Make issue shepherding phase-aware for community contributions

**What problem would this solve?**

The GitHub-to-Buzz automation currently assigns a core-team member to shepherd an Inbox issue. Once the issue advances, leaving that person assigned can make a community issue look claimed and discourage contributors from proposing a design or implementation.

At the same time, community contributions still need a core-team member to lead the design discussion and verify an incoming pull request. Assignment should communicate the role appropriate to the current project phase without repeatedly overwriting intentional manual assignments.

So the proposal would be:

- **Inbox:** continue assigning a core-team shepherd using the existing GitHub-to-Buzz manager.
- **Accepted / design:** when an issue enters this phase, unassign the current shepherd once and remember them as the previous shepherd. Do not repeatedly clear assignments while the issue remains in this phase, so a core member can intentionally self-assign afterward.
- After the transition to **Accepted / design**, the first new human comment on the issue should cause the agent to select and assign a core-team member to lead the design discussion. Comments that predate the transition must not trigger assignment. Bot and system comments should be ignored.
- When selecting that design shepherd, the agent should consider continuity with the previous shepherd, subject-matter interest, and current normalized workload. The previous shepherd should receive a continuity preference, not an automatic assignment.
- **Ready:** when an issue enters this phase, unassign its current shepherd once so it is visibly available, then wait for an incoming pull request.
- When a pull request is linked to the issue, move the issue to **Verification**. Have the agent select a core-team verifier using previous-shepherd continuity, subject-matter interest, and normalized workload, then assign the same person to both the issue and the pull request.
- Reflect the resulting phase and assignments in the corresponding Buzz channel through the existing synchronization flow.
- Keep hourly reconciliation idempotent: rerunning it must not clear later intentional assignments, repeat phase transitions, or process the same comment or pull request more than once.