v0.8.0

feat(cli,api,store): binding CRUD, multi-destination cred add, cred update (#24)

- New sluice binding add/list/update/remove CLI subcommands and PATCH /api/bindings/{id}
- New sluice cred update CLI subcommand and PATCH /api/credentials/{name}
- sluice cred add --destination is now repeatable
- New migration 000005: case-insensitive UNIQUE index on bindings(credential, LOWER(destination))
  with operator-actionable conflict detection on upgrade
- Bindings of the same credential can share env_var
- Sentinel errors for proper API HTTP status mapping (409/404/400/500)
- CAS-protected credential rollback against concurrent writers
- Authoritative cred type detection via credential_meta