fix(adr): unify manage_adr onto the SQLite store shared with the UI (#256)

manage_adr (MCP/CLI) wrote/read ADRs as a file at <root>/.codebase-memory/adr.md,
while the UI /api/adr endpoints used the SQLite project_summaries table via
cbm_store_adr_get/store. Writes through one interface were invisible to the other.

Route manage_adr through cbm_store_adr_get/store (the same backend the UI uses),
so update/get/sections all operate on the shared store. Add a one-time migration:
if the store has no ADR but the legacy adr.md file exists, import it on first
access so nothing is lost on upgrade. The legacy file reader is retained solely
for that migration.