fix(migration-legacy): don't classify valid-JSON non-object lines as corrupt

`analyzeContextContent` documents `corrupt` as "every non-blank line
failed to parse" (disk damage / truncated write). But a line that
JSON.parse accepted yet was a scalar/array (e.g. `42`, `"hi"`, `[]`) hit
the object-shape `continue` before `hadParseableLine` was set, so a
context of all valid-JSON non-objects was misreported as `corrupt` and
surfaced as a migration failure instead of an empty/skipped session.

Mark the line as parseable as soon as JSON.parse succeeds, before the
shape check — matching the documented contract and the existing
"at least one line parses → empty" test intent. Genuinely truncated
lines still fail JSON.parse and remain `corrupt`.

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

Author: Dmatut7

packages/migration-legacy/src/sessions/translator.ts

@@ -49,8 +49,12 @@ export function analyzeContextContent(lines: readonly string[]): ContextContent
     } catch {
       continue;
     }
-    if (typeof parsed !== 'object' || parsed === null) continue;
+    // A line that JSON.parse accepts has "parsed" per the corrupt contract
+    // above, even when it is a scalar/array rather than an object. Mark it
+    // before the shape check so an all-valid-JSON-but-no-object context is
+    // classified 'empty' (cleared session), not 'corrupt' (disk damage).
     hadParseableLine = true;
+    if (typeof parsed !== 'object' || parsed === null) continue;
     const role = (parsed as Record<string, unknown>)['role'];
     if (typeof role === 'string' && USABLE_ROLES.has(role)) return 'real';
   }

packages/migration-legacy/test/sessions/translator.test.ts

@@ -170,4 +170,12 @@ describe('analyzeContextContent', () => {
       ]),
     ).toBe('empty');
   });
+
+  it("'empty' (not 'corrupt') when lines are valid JSON scalars/arrays, not objects", () => {
+    // These lines parse successfully, so they are not "disk damage". Per the
+    // corrupt contract (every non-blank line *failed to parse*), they must be
+    // classified 'empty', not 'corrupt'.
+    expect(analyzeContextContent(['42', '"hi"', 'true'])).toBe('empty');
+    expect(analyzeContextContent(['[]'])).toBe('empty');
+  });
 });