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/migration.md

@@ -73,7 +73,7 @@ the structural binding.
 ## Migration checklist per skill
 
 1. Strip project-specific paths (`docs/specs`, `docs/plans`,
-   `design/INDEX.md`, `crates/`, `bench/`).
+   `docs/design/INDEX.md`, `crates/`, `bench/`).
 2. Strip project-specific commands (`cargo build`,
    `bench/check.py`).
 3. Replace literals with profile-slot references in prose.
@@ -91,7 +91,7 @@ the structural binding.
 1. Drop the `ailang-` prefix from the `name:` frontmatter
    field. The skill path is the disambiguator.
 2. Replace hardcoded standing-reading paths
-   (`design/INDEX.md`, etc.) with a reference to the profile's
+   (`docs/design/INDEX.md`, etc.) with a reference to the profile's
    `standing_reading` section.
 3. Replace project-specific Iron Law clauses with the universal
    discipline constants; project-specific clauses go to the