fix(preamble): progress notes on phase changes, not a [PROGRESS] cadence (s01 M2)

Context Health told the model to periodically write a tagged [PROGRESS]
summary. No code reads that tag; the line now says when to tell the user
(finishing a phase or changing direction). Loop detection stays.

W7a pin: test/terse-build.test.ts:103 pinned 'PROGRESS'; it now checks the
default build emits a non-empty Context Health body (terse still emits '').

test/terse-build.test.ts

@@ -100,7 +100,7 @@ describe('terse build — per-resolver behavior', () => {
     test('default: emits full section', () => {
       const out = generateContextHealth(makeCtx('default'));
       expect(out).toContain('## Context Health');
-      expect(out).toContain('PROGRESS');
+      expect(out.replace(/^## Context Health.*$/m, '').trim().length).toBeGreaterThan(80);
     });
 
     test('terse: returns empty string', () => {