Separate triage columns by severity

### Description

**Problem:** Issues with no severity label default to the most aggressive deadline (2 business days, same as severity/important). The issue must already have `status/triage` to appear in this column, but if it's missing a `severity/*` label, it gets the fastest overdue clock. If someone files a private triage issue on Friday without adding a severity label, it shows as overdue by Wednesday.

**Proposal:** Split the private triage column into separate columns per severity — including one for missing severity — so each can have its own threshold.

### Value Proposition

Different severities have different triage urgency. Separate columns give a clearer picture of where things stand without lumping everything into one bucket.

### Goals

- Add separate triage columns for each severity level plus a "missing severity" column
- Set different overdue thresholds per column

### Non-Goals

_No response_

### Discussion

_No response_

### Notes

Issues with no severity set could still be critical, so the missing-severity column should still encourage prompt labeling. Severity should be treated as fluid — set an initial estimate early and refine it later rather than treating it as a one-time decision.

No files, tests, or entry points are named. Start by locating the private triage column configuration and the definitions for status/triage and severity/* labels. Done means separate columns exist for each severity and missing severity, with independently configured overdue thresholds.

Written by the indexing model from the issue text.