docs: default design_ledger path to docs/design/INDEX.md

Align the design_ledger profile-slot default and its illustrative
references with the docs/ layout used by the other path slots.
Historical specs/plans are left untouched as cycle records.

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

docs/profile-schema.md

@@ -12,7 +12,7 @@ lowercase snake_case. Lists are YAML sequences.
 | `spec_dir`          | string | `docs/specs`           | Where the brainstorm skill writes specs. |
 | `plan_dir`          | string | `docs/plans`           | Where the planner skill writes plans. |
 | `glossary`          | string | (unset)                | Canonical-nomenclature file (optional). If set, it is standing reading for every role — no separate `standing_reading.always` entry is needed; unset is a documented no-op. See `glossary-convention.md`. |
-| `design_ledger`     | string | `design/INDEX.md`      | Canonical specification index (optional — projects without a design ledger can omit). |
+| `design_ledger`     | string | `docs/design/INDEX.md` | Canonical specification index (optional — projects without a design ledger can omit). |
 | `design_contracts`  | string | `design/contracts`     | Directory of prose-authoritative contracts (optional). |
 | `design_models`     | string | `design/models`        | Directory of onboarding whitepapers (optional). |
 | `code_roots`        | list   | `[src]`                | Code directories the architect / quality reviewer walk. |