pm close should support --duplicate-of flag for proper duplicate item tracking #183

Problem

When closing duplicate PM items, there is no standardized way to track which item is the canonical/original. The close reason text is freeform, making it impossible to programmatically trace duplicate chains.

Current Behavior

pm close pm-xyz2 'Duplicate of pm-abc1'

This creates a close entry in history but doesn't establish a machine-readable link between the duplicate and the original.

Desired Behavior

pm close pm-xyz2 --duplicate-of pm-abc1

This should:

1. Set a duplicate_of reference field on the closed item
2. Generate standardized close reason: 'Duplicate of pm-abc1'
3. Add a comment on the original item: 'pm-xyz2 closed as duplicate'
4. Support pm deps to trace duplicate relationships
5. Validate that the target item exists and is not itself a duplicate

Impact

In our audit we found 2 duplicate items and there is no structured way to mark them. Across 1075 items, duplicate detection and tracking is essential for PM hygiene.