Summary

For a no-op `pm update` (setting a field to its current value), the JSON output incorrectly reports the field in `changed_fields`, while the TOON output reports an empty `changed_fields`. The two output formats disagree about what changed. Additionally, `changed_field_count` is always `null` in --json output (the CLI computes it for TOON but never populates it in the JSON object).

Steps to reproduce

```bash
pm init --path /tmp/t && cd /tmp/t
ID=$(pm create --type Task --title t --path /tmp/t | grep -o 'pm-[a-z0-9]*' | head -1)

no-op: set title to its existing value

pm update $ID --title t --json --path /tmp/t | python3 -c "import sys,json;d=json.load(sys.stdin);print('changed_fields=',d.get('changed_fields'));print('changed_field_count=',d.get('changed_field_count'))"

-> changed_fields= ['title'] (phantom) ; changed_field_count= None

pm update $ID --title t --path /tmp/t # TOON: "changed_fields:" with empty list
```

Expected

• A no-op update should report an empty `changed_fields` in BOTH formats (or the TOON empty behavior is correct and JSON should match).
• `changed_field_count` should be a number (e.g. 0 for no-op, 1 for a real change) in --json output, not `null`.

Actual

• JSON reports `changed_fields: ['title']` for an unchanged value (phantom change); TOON reports empty.
• `changed_field_count` is `null` in JSON regardless of whether a field actually changed.

Impact

Agents/CI that branch on `changed_fields` or `changed_field_count` from --json output are misled into thinking a change occurred when it did not, and cannot reliably count changes.

Environment

pm version: 2026.7.12, OS: linux, node: v25.9.0

Suspected area

`update` command mutation result serialization (changed_fields diffing + changed_field_count population in JSON mode).