Add cancelled and superseded terminal work statuses

Acceptance Criteria

• work rune accepts cancelled and superseded status values (schema tags/work.ts, enums.ts VALID_STATUS, and MCP plan.update input schema)
• work rune accepts an optional single-valued supersedes attribute referencing an entity ID (added to ALLOWED_ATTRS.work)
• cancelled / superseded are registered in TERMINAL_STATUSES.work but not ACHIEVING_STATUSES.work
• cancelled / superseded work items are excluded from plan next, from milestone progress numerators, and from plan-progress achieved counts
• config.ts work sentimentMap gains muted/caution entries for cancelled and superseded; render-pipeline orderings sort them into the terminal tail
• supersedes produces a superseded-by / supersedes relationship edge in relationships.ts
• plan validate warns on a superseded work item with no supersedes, and no longer warns about a ## Resolution present on a cancelled / superseded item
• Tests cover: schema acceptance, exclusion from next/progress, the supersedes-missing warning, and the resolution-on-terminal carve-out