Fix workspace status aggregation logic

[milldr]

What

Two fixes to workspace status aggregation:

1. Add a spec-level skip shell command that excludes repos from workspace aggregation (default check detects default-branch repos like main/master)
2. Make the default: true status passive in aggregation — it no longer competes with active statuses like in-progress or in-review

Why

Default-branch repos always resolved to open (no PRs exist for main), dragging the entire workspace status down. And the fallback open status dominated active work signals, making workspace status misleading for multi-repo workspaces.

Ref

• internal/status/types.go — new Skip field on SpecBody
• internal/status/resolve.go — updated aggregation logic
• internal/status/spec.go — default skip check for default-branch detection
• internal/status/resolve_test.go — 5 new test cases