feat(audit): declarative field-change activity via trackHistory (ADR-0052 §5b)

The activity writer already captures the field diff but renders a flat
"Updated <object>" summary. Add a per-field `trackHistory` flag (spec) and wire
the audit-writer to render tracked changes legibly — "Stage: Proposal → Closed
Won" — using the field label and select option labels. Opt-in per field (cf.
Salesforce Feed Tracking / ServiceNow field auditing / Dataverse column
auditing). Reintroduces the pruned `auditTrail` concept WITH a runtime consumer,
satisfying enforce-or-remove (ADR-0049).

Apps can now DECLARE their timeline instead of hand-coding hooks/flows that
insert sys_activity rows.

- spec: trackHistory?: boolean on the field schema
- plugin-audit: renderTrackedChangeSummary + getFieldDefs; update summary uses
  it, falling back to the generic text when no tracked field changed
- tests: 2 new cases (renders label/option diff; falls back when untracked)
- docs: ADR-0052 §5b "Declarative activity, not per-app code"

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang