docs(schema): Rationale constraint table missing secondary indexes

[egouilliard-leyton]

Context

Discovered during review of #272 fix (stale Cypher patterns).

Description

The constraint table in codegraph/docs/schema.md line 49 shows:

| `:Rationale` | `id` | — |

But the loader (loader.py:173-174) creates two secondary indexes:

• rationale_source on source_file
• rationale_decision on decision_title

The table should read:

| `:Rationale` | `id` | `source_file`, `decision_title` |

Suggested approach

One-line fix in codegraph/docs/schema.md.